types of fetal presentation ultrasound

Variation in fetal presentation

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• Delivery presentations
• Variation in delivary presentation
• Abnormal fetal presentations

There can be many variations in the fetal presentation which is determined by which part of the fetus is projecting towards the internal cervical os . This includes:

cephalic presentation : fetal head presenting towards the internal cervical os, considered normal and occurs in the vast majority of births (~97%); this can have many variations which include

left occipito-anterior (LOA)

left occipito-posterior (LOP)

left occipito-transverse (LOT)

right occipito-anterior (ROA)

right occipito-posterior (ROP)

right occipito-transverse (ROT)

straight occipito-anterior

straight occipito-posterior

breech presentation : fetal rump presenting towards the internal cervical os, this has three main types

frank breech presentation  (50-70% of all breech presentation): hips flexed, knees extended (pike position)

complete breech presentation  (5-10%): hips flexed, knees flexed (cannonball position)

footling presentation  or incomplete (10-30%): one or both hips extended, foot presenting

other, e.g one leg flexed and one leg extended

shoulder presentation

cord presentation : umbilical cord presenting towards the internal cervical os

• 1. Fox AJ, Chapman MG. Longitudinal ultrasound assessment of fetal presentation: a review of 1010 consecutive cases. Aust N Z J Obstet Gynaecol. 2006;46 (4): 341-4. doi:10.1111/j.1479-828X.2006.00603.x - Pubmed citation
• 2. Merz E, Bahlmann F. Ultrasound in obstetrics and gynecology. Thieme Medical Publishers. (2005) ISBN:1588901475. Read it at Google Books - Find it at Amazon

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Fetal Presentation, Position, and Lie (Including Breech Presentation)

• Variations in Fetal Position and Presentation |

During pregnancy, the fetus can be positioned in many different ways inside the mother's uterus. The fetus may be head up or down or facing the mother's back or front. At first, the fetus can move around easily or shift position as the mother moves. Toward the end of the pregnancy the fetus is larger, has less room to move, and stays in one position. How the fetus is positioned has an important effect on delivery and, for certain positions, a cesarean delivery is necessary. There are medical terms that describe precisely how the fetus is positioned, and identifying the fetal position helps doctors to anticipate potential difficulties during labor and delivery.

Presentation refers to the part of the fetus’s body that leads the way out through the birth canal (called the presenting part). Usually, the head leads the way, but sometimes the buttocks (breech presentation), shoulder, or face leads the way.

Position refers to whether the fetus is facing backward (occiput anterior) or forward (occiput posterior). The occiput is a bone at the back of the baby's head. Therefore, facing backward is called occiput anterior (facing the mother’s back and facing down when the mother lies on her back). Facing forward is called occiput posterior (facing toward the mother's pubic bone and facing up when the mother lies on her back).

Lie refers to the angle of the fetus in relation to the mother and the uterus. Up-and-down (with the baby's spine parallel to mother's spine, called longitudinal) is normal, but sometimes the lie is sideways (transverse) or at an angle (oblique).

For these aspects of fetal positioning, the combination that is the most common, safest, and easiest for the mother to deliver is the following:

Head first (called vertex or cephalic presentation)

Facing backward (occiput anterior position)

Spine parallel to mother's spine (longitudinal lie)

Neck bent forward with chin tucked

Arms folded across the chest

If the fetus is in a different position, lie, or presentation, labor may be more difficult, and a normal vaginal delivery may not be possible.

Variations in fetal presentation, position, or lie may occur when

The fetus is too large for the mother's pelvis (fetopelvic disproportion).

The uterus is abnormally shaped or contains growths such as fibroids .

The fetus has a birth defect .

There is more than one fetus (multiple gestation).

Position and Presentation of the Fetus

Variations in Fetal Position and Presentation

Some variations in position and presentation that make delivery difficult occur frequently.

Occiput posterior position

In occiput posterior position (sometimes called sunny-side up), the fetus is head first (vertex presentation) but is facing forward (toward the mother's pubic bone—that is, facing up when the mother lies on her back). This is a very common position that is not abnormal, but it makes delivery more difficult than when the fetus is in the occiput anterior position (facing toward the mother's spine—that is facing down when the mother lies on her back).

When a fetus faces up, the neck is often straightened rather than bent,which requires more room for the head to pass through the birth canal. Delivery assisted by a vacuum device or forceps or cesarean delivery may be necessary.

Breech presentation

In breech presentation, the baby's buttocks or sometimes the feet are positioned to deliver first (before the head).

When delivered vaginally, babies that present buttocks first are more at risk of injury or even death than those that present head first.

The reason for the risks to babies in breech presentation is that the baby's hips and buttocks are not as wide as the head. Therefore, when the hips and buttocks pass through the cervix first, the passageway may not be wide enough for the head to pass through. In addition, when the head follows the buttocks, the neck may be bent slightly backwards. The neck being bent backward increases the width required for delivery as compared to when the head is angled forward with the chin tucked, which is the position that is easiest for delivery. Thus, the baby’s body may be delivered and then the head may get caught and not be able to pass through the birth canal. When the baby’s head is caught, this puts pressure on the umbilical cord in the birth canal, so that very little oxygen can reach the baby. Brain damage due to lack of oxygen is more common among breech babies than among those presenting head first.

In a first delivery, these problems may occur more frequently because a woman’s tissues have not been stretched by previous deliveries. Because of risk of injury or even death to the baby, cesarean delivery is preferred when the fetus is in breech presentation, unless the doctor is very experienced with and skilled at delivering breech babies or there is not an adequate facility or equipment to safely perform a cesarean delivery.

Breech presentation is more likely to occur in the following circumstances:

Labor starts too soon (preterm labor).

The uterus is abnormally shaped or contains abnormal growths such as fibroids .

Other presentations

In face presentation, the baby's neck arches back so that the face presents first rather than the top of the head.

In brow presentation, the neck is moderately arched so that the brow presents first.

Usually, fetuses do not stay in a face or brow presentation. These presentations often change to a vertex (top of the head) presentation before or during labor. If they do not, a cesarean delivery is usually recommended.

In transverse lie, the fetus lies horizontally across the birth canal and presents shoulder first. A cesarean delivery is done, unless the fetus is the second in a set of twins. In such a case, the fetus may be turned to be delivered through the vagina.

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• General Ultrasound Background

Ultrasound determination of fetal lie and presentation

This leaflet is to help you understand the ultrasound determination of fetal lie and presentation.

What is fetal lie?

Fetal lie refers to the orientation of the fetus in the womb in relation to the mother and uterus. A vertical orientation (with the baby's spine aligned parallel to the mother's spine, known as a longitudinal lie) is typical, although occasionally, the position may be horizontal (transverse) or at an incline (oblique).

What is the most frequent fetal lie?

The most frequent fetal lie is the longitudinal lie, accounting for 99% of pregnancies after 28 weeks’ gestation.

How is fetal lie evaluated?

Traditionally, your obstetric caregiver using standardized manual palpation techniques, commonly known as Leopold’s maneuvers, evaluates the fetal lie. However, these maneuvers are not consistently precise. Conversely, ultrasound scanning offers an objective and reliable approach to determining fetal lie, now widely regarded as the gold standard.

What is fetal presentation?

The fetal presentation describes the fetal part that is lowest in the maternal abdomen. In case of labor, it is the lowest fetal part in the birth canal. Many fetal presentations are possible:

• Cephalic presentation: the fetal head is the lowest fetal part. This is by far the most common presentation at term of pregnancy and in labor.
• Breech: the fetal buttock or feet are the lowest fetal part.
• Shoulder: the fetal shoulder is the lowest fetal part.
• Compound: a combination of more than one fetal structure lies closest to the pelvic inlet.

How can the fetal presentation be evaluated?

Traditionally, fetal presentation is assessed using clinical examination. This can include maternal abdominal palpation and vaginal examination. The latter is particularly useful in labor, when cervical dilatation allows the examiner to directly feel the lowermost fetal part.

In case of uncertainty, mainly when the fetal lowermost part is difficult to appreciate by abdominal or vaginal examination, fetal presentation can be easily evaluated by transabdominal ultrasound.

Last updated April 2024

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• Fetal presentation before birth

The way a baby is positioned in the uterus just before birth can have a big effect on labor and delivery. This positioning is called fetal presentation.

Babies twist, stretch and tumble quite a bit during pregnancy. Before labor starts, however, they usually come to rest in a way that allows them to be delivered through the birth canal headfirst. This position is called cephalic presentation. But there are other ways a baby may settle just before labor begins.

Following are some of the possible ways a baby may be positioned at the end of pregnancy.

Head down, face down

When a baby is head down, face down, the medical term for it is the cephalic occiput anterior position. This the most common position for a baby to be born in. With the face down and turned slightly to the side, the smallest part of the baby's head leads the way through the birth canal. It is the easiest way for a baby to be born.

Head down, face up

When a baby is head down, face up, the medical term for it is the cephalic occiput posterior position. In this position, it might be harder for a baby's head to go under the pubic bone during delivery. That can make labor take longer.

Most babies who begin labor in this position eventually turn to be face down. If that doesn't happen, and the second stage of labor is taking a long time, a member of the health care team may reach through the vagina to help the baby turn. This is called manual rotation.

In some cases, a baby can be born in the head-down, face-up position. Use of forceps or a vacuum device to help with delivery is more common when a baby is in this position than in the head-down, face-down position. In some cases, a C-section delivery may be needed.

Frank breech

When a baby's feet or buttocks are in place to come out first during birth, it's called a breech presentation. This happens in about 3% to 4% of babies close to the time of birth. The baby shown below is in a frank breech presentation. That's when the knees aren't bent, and the feet are close to the baby's head. This is the most common type of breech presentation.

If you are more than 36 weeks into your pregnancy and your baby is in a frank breech presentation, your health care professional may try to move the baby into a head-down position. This is done using a procedure called external cephalic version. It involves one or two members of the health care team putting pressure on your belly with their hands to get the baby to roll into a head-down position.

If the procedure isn't successful, or if the baby moves back into a breech position, talk with a member of your health care team about the choices you have for delivery. Most babies in a frank breech position are born by planned C-section.

Complete and incomplete breech

A complete breech presentation, as shown below, is when the baby has both knees bent and both legs pulled close to the body. In an incomplete breech, one or both of the legs are not pulled close to the body, and one or both of the feet or knees are below the baby's buttocks. If a baby is in either of these positions, you might feel kicking in the lower part of your belly.

If you are more than 36 weeks into your pregnancy and your baby is in a complete or incomplete breech presentation, your health care professional may try to move the baby into a head-down position. This is done using a procedure called external cephalic version. It involves one or two members of the health care team putting pressure on your belly with their hands to get the baby to roll into a head-down position.

If the procedure isn't successful, or if the baby moves back into a breech position, talk with a member of your health care team about the choices you have for delivery. Many babies in a complete or incomplete breech position are born by planned C-section.

When a baby is sideways — lying horizontal across the uterus, rather than vertical — it's called a transverse lie. In this position, the baby's back might be:

• Down, with the back facing the birth canal.
• Sideways, with one shoulder pointing toward the birth canal.
• Up, with the hands and feet facing the birth canal.

Although many babies are sideways early in pregnancy, few stay this way when labor begins.

If your baby is in a transverse lie during week 37 of your pregnancy, your health care professional may try to move the baby into a head-down position. This is done using a procedure called external cephalic version. External cephalic version involves one or two members of your health care team putting pressure on your belly with their hands to get the baby to roll into a head-down position.

If the procedure isn't successful, or if the baby moves back into a transverse lie, talk with a member of your health care team about the choices you have for delivery. Many babies who are in a transverse lie are born by C-section.

If you're pregnant with twins and only the twin that's lower in the uterus is head down, as shown below, your health care provider may first deliver that baby vaginally.

Then, in some cases, your health care team may suggest delivering the second twin in the breech position. Or they may try to move the second twin into a head-down position. This is done using a procedure called external cephalic version. External cephalic version involves one or two members of the health care team putting pressure on your belly with their hands to get the baby to roll into a head-down position.

Your health care team may suggest delivery by C-section for the second twin if:

• An attempt to deliver the baby in the breech position is not successful.
• You do not want to try to have the baby delivered vaginally in the breech position.
• An attempt to move the baby into a head-down position is not successful.
• You do not want to try to move the baby to a head-down position.

In some cases, your health care team may advise that you have both twins delivered by C-section. That might happen if the lower twin is not head down, the second twin has low or high birth weight as compared to the first twin, or if preterm labor starts.

• Landon MB, et al., eds. Normal labor and delivery. In: Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Elsevier; 2021. https://www.clinicalkey.com. Accessed May 19, 2023.
• Holcroft Argani C, et al. Occiput posterior position. https://www.updtodate.com/contents/search. Accessed May 19, 2023.
• Frequently asked questions: If your baby is breech. American College of Obstetricians and Gynecologists https://www.acog.org/womens-health/faqs/if-your-baby-is-breech. Accessed May 22, 2023.
• Hofmeyr GJ. Overview of breech presentation. https://www.updtodate.com/contents/search. Accessed May 22, 2023.
• Strauss RA, et al. Transverse fetal lie. https://www.updtodate.com/contents/search. Accessed May 22, 2023.
• Chasen ST, et al. Twin pregnancy: Labor and delivery. https://www.updtodate.com/contents/search. Accessed May 22, 2023.
• Cohen R, et al. Is vaginal delivery of a breech second twin safe? A comparison between delivery of vertex and non-vertex second twins. The Journal of Maternal-Fetal & Neonatal Medicine. 2021; doi:10.1080/14767058.2021.2005569.
• Marnach ML (expert opinion). Mayo Clinic. May 31, 2023.

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Breech, posterior, transverse lie: What position is my baby in?

Fetal presentation, or how your baby is situated in your womb at birth, is determined by the body part that's positioned to come out first, and it can affect the way you deliver. At the time of delivery, 97 percent of babies are head-down (cephalic presentation). But there are several other possibilities, including feet or bottom first (breech) as well as sideways (transverse lie) and diagonal (oblique lie).

Fetal presentation and position

During the last trimester of your pregnancy, your provider will check your baby's presentation by feeling your belly to locate the head, bottom, and back. If it's unclear, your provider may do an ultrasound or an internal exam to feel what part of the baby is in your pelvis.

Fetal position refers to whether the baby is facing your spine (anterior position) or facing your belly (posterior position). Fetal position can change often: Your baby may be face up at the beginning of labor and face down at delivery.

Here are the many possibilities for fetal presentation and position in the womb.

Medical illustrations by Jonathan Dimes

Head down, facing down (anterior position)

A baby who is head down and facing your spine is in the anterior position. This is the most common fetal presentation and the easiest position for a vaginal delivery.

This position is also known as "occiput anterior" because the back of your baby's skull (occipital bone) is in the front (anterior) of your pelvis.

Head down, facing up (posterior position)

In the posterior position , your baby is head down and facing your belly. You may also hear it called "sunny-side up" because babies who stay in this position are born facing up. But many babies who are facing up during labor rotate to the easier face down (anterior) position before birth.

Posterior position is formally known as "occiput posterior" because the back of your baby's skull (occipital bone) is in the back (posterior) of your pelvis.

Frank breech

In the frank breech presentation, both the baby's legs are extended so that the feet are up near the face. This is the most common type of breech presentation. Breech babies are difficult to deliver vaginally, so most arrive by c-section .

Some providers will attempt to turn your baby manually to the head down position by applying pressure to your belly. This is called an external cephalic version , and it has a 58 percent success rate for turning breech babies. For more information, see our article on breech birth .

Complete breech

A complete breech is when your baby is bottom down with hips and knees bent in a tuck or cross-legged position. If your baby is in a complete breech, you may feel kicking in your lower abdomen.

Incomplete breech

In an incomplete breech, one of the baby's knees is bent so that the foot is tucked next to the bottom with the other leg extended, positioning that foot closer to the face.

Single footling breech

In the single footling breech presentation, one of the baby's feet is pointed toward your cervix.

Double footling breech

In the double footling breech presentation, both of the baby's feet are pointed toward your cervix.

Transverse lie

In a transverse lie, the baby is lying horizontally in your uterus and may be facing up toward your head or down toward your feet. Babies settle this way less than 1 percent of the time, but it happens more commonly if you're carrying multiples or deliver before your due date.

If your baby stays in a transverse lie until the end of your pregnancy, it can be dangerous for delivery. Your provider will likely schedule a c-section or attempt an external cephalic version , which is highly successful for turning babies in this position.

Oblique lie

In rare cases, your baby may lie diagonally in your uterus, with his rump facing the side of your body at an angle.

Like the transverse lie, this position is more common earlier in pregnancy, and it's likely your provider will intervene if your baby is still in the oblique lie at the end of your third trimester.

Was this article helpful?

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BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies .

Ahmad A et al. 2014. Association of fetal position at onset of labor and mode of delivery: A prospective cohort study. Ultrasound in obstetrics & gynecology 43(2):176-182. https://www.ncbi.nlm.nih.gov/pubmed/23929533 Opens a new window [Accessed September 2021]

Gray CJ and Shanahan MM. 2019. Breech presentation. StatPearls.  https://www.ncbi.nlm.nih.gov/books/NBK448063/ Opens a new window [Accessed September 2021]

Hankins GD. 1990. Transverse lie. American Journal of Perinatology 7(1):66-70.  https://www.ncbi.nlm.nih.gov/pubmed/2131781 Opens a new window [Accessed September 2021]

Medline Plus. 2020. Your baby in the birth canal. U.S. National Library of Medicine. https://medlineplus.gov/ency/article/002060.htm Opens a new window [Accessed September 2021]

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What Are the Different Fetal Positions?

• Medical Author: Karthik Kumar, MBBS
• Medical Reviewer: Pallavi Suyog Uttekar, MD

5 Types of Fetal Positions and Presentations

• Comments **COMMENTSTAGLIST**
• More **OTHERTAGLIST**

The relationship between your baby's backbone and your backbone when your baby is in-utero is called the fetal position. Your baby can be in a variety of fetal positions, some make birth easier than others.

• Longitudinal position: The fetus’ and mother’s backbones are parallel to each other in this position.
• Transverse position: In this posture, the fetus’ backbone is at a right angle to the mother's backbone.
• Oblique position: The inclination angle of the fetus backbone is more than 0 and less than 90 degrees of the mother's backbone in this position.

Most people, however, confuse fetal position with the fetal presentation.

• Fetal position refers to whether the fetus is facing backward (facing the woman's back when she lies down) or forward (facing the woman's abdomen when she lies down).
• Fetal presentation is the body part of the baby that leads the way out of the birth canal.

The fetal position and presentation of your baby may influence the difficulty of your delivery. The baby may drop down into the pelvis before the due date. Here are some of the different positions and presentations your baby can get into while you are preparing for childbirth .

During pregnancy and when preparing for childbirth , there are exercises moms can do when the baby is active to get it in the optimal fetal position, which is known as baby spinning. Starting at the 35th week of pregnancy, talk to your doctor about maternal positioning.

Occiput anterior (OA) or vertex presentation

This is the optimal fetal positioning for childbirth . The baby enters the pelvis with their head down and chin tucked to the chest, facing the mother's back. The head points to the birth canal in this position.

There are two more presentations in OA:

• The baby will remain in the OA position, but the face, rather than the head, will be pointing toward the birth canal.
• This occurs when the chin is not tucked against the chest and instead points outward.
• During a vaginal examination, the doctor can detect this position by feeling the baby's bony jaws and mouth.
• In brow presentation, the baby will be in the OA position with their forehead pointing toward the birth canal. The doctor can feel the anterior fontanelle and the orbits of the forehead during the vaginal examination.
• One arm lies along with the head, pointing toward the birth canal.
• The arms may slide back during the delivery process, but if they do not, extra care must be taken to safely remove the baby.

Occiput posterior (OP)

• The baby enters the pelvis with its head down but facing the mother's front or abdomen.
• In general, approximately 10 to 34 percent of babies remain in the OP position during the first stage of labor before shifting to the optimal (OA) position.
• However, some babies remain in this position, which can make labor difficult and necessitate an emergency Cesarean delivery.
• This fetal position can cause labor to be prolonged, resulting in instrumental interventions, severe perineal tears, or Cesarean delivery.

The cephalic presentation or head-first positions are referred to as OA and OP.

Occiput transverse (OT)

• In the womb, the baby is lying sideways, and if they do not turn to the optimal position in time for birth, a Cesarean delivery is required.
• During a vaginal examination, the doctor may feel the shoulder, arm, elbow, or hand protruding into the vagina.
• This baby position increases the risk of umbilical cord prolapse, which occurs when the umbilical cord protrudes before the baby.
• Cord prolapse can occur in about one percent of babies in the transverse position, which is a medical emergency that necessitates an immediate Cesarean delivery.
• In some cases, assisted delivery is performed by manually rotating the baby or using forceps or a vacuum to position the baby in the ideal position.

Umbilical cord presentation

• During this time, the umbilical cord is the first to emerge from the birth canal.
• The condition of the uterine membrane, however, distinguishes umbilical cord presentation from prolapse.
• A cord presentation occurs when the umbilical cord enters the birth canal before the water breaks, whereas a cord prolapse occurs after the water breaks, necessitating an emergency Cesarean delivery.

Breech position

The infant is positioned with its buttocks directed toward the birth canal, resulting in the following types of breech positions:

• The buttocks are pointing toward the birth canal, with the legs folded at the knees and the feet close to the buttocks.
• In a vaginal delivery, this position increases the risk of an umbilical cord loop. Furthermore, the cord may pass through the cervix before the head, injuring the baby.
• The buttocks are pointing toward the birth canal with the legs straight up and the feet reaching the head.
• This can result in an umbilical cord loop, which can injure the baby during vaginal birth.
• The baby's buttocks are pointing down, and one of their feet is pointing toward the birthing canal.
• This can result in an umbilical cord prolapse, which can cut off the fetus' blood supply and oxygen supply.

A clinical examination of the abdomen, a vaginal examination, or an ultrasound examination is used to determine the position and presentation of the fetus during pregnancy.

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Fetal Ultrasound

• • An imaging study in which high-frequency sound waves create pictures of a fetus inside the uterus
• • For viewing the fetus during pregnancy
• • It is also utilized to guide procedures such as amniocentesis or chorionic villus sampling
• • Involves Ob/Gyn and Maternal-Fetal Medicine
• Prenatal Ultrasound Scan

What is ultrasound?

Who performs an ultrasound, how do ultrasounds assist pregnancy, in what other ways is ultrasound used, how do you prepare for an ultrasound, how is yale medicine's use of ultrasound technology unique.

Since the mid-20th century, ultrasound has been used extensively to create a "picture" of what's happening inside our bodies. For pregnant women, it is an indispensable aspect to care. 

At Yale Medicine Maternal-Fetal Medicine , our physicians and sonographers perform more than 16,000 ultrasounds a year. 

"Experience counts. This is the only way we can approach the fetus like a patient. With sophisticated ultrasound, we now have ways to identify fetal problems and guide treatment," says Joshua Copel, MD , a Yale Medicine expert in prenatal diagnosis and fetal therapy. "If a baby has an abnormal amount of fluid in the chest, we can use an ultrasound-guided needle to drain it out. Or we can deliver blood to the umbilical cord. The first of these types of transfusions were done here at Yale in 1984."

Think of ultrasound—sometimes called a sonogram—as sonar for the body. A small instrument called a “transducer” transmits high-frequency sound waves, bouncing them off structures in the body and using the reflected sound waves to paint a picture.

The sound waves can determine the distance, size, shape and consistency of an object. A computer compiles these results to create an image of the object, be it a fetus, an adult heart, or a tumor in the kidney.

Specially trained physicians and sonographers perform ultrasounds by spreading gel on the skin adjacent to the body part to be imaged. (The gel helps to conduct the sound waves and makes it easier to move the transducer over the skin.) Most ultrasound procedures are painless and non-invasive.

Ultrasound imaging has been used for decades and has an excellent safety record. It’s important to remember that ultrasound uses sound waves; it does not use ionizing radiation, like X-rays, which can cause harm after prolonged exposure

Ultrasound is the most widely used medical imaging method for viewing a fetus during pregnancy. It is also utilized to guide procedures such as  amniocentesis or chorionic villus sampling .

An ultrasound may be done at almost any time during a pregnancy, even as early as about five weeks after conception. During the first trimester, it can confirm a viable pregnancy and heartbeat, measure the distance from the head to the rear end in order to confirm gestational age, and identify problems such as molar or ectopic pregnancies. It can also identify some uterine or pelvic problems in the mother.

In the second trimester, ultrasound can pinpoint fetal malformations, confirm multiple pregnancies (though this is ideally done in the first trimester), monitor levels of amniotic fluid and evaluate how well the fetus is doing.

In the third trimester, ultrasound can show the placenta's placement, how the fetus is moving and presented for birth, and measure how well the baby is growing. 

• Abdominal ultrasound can create images of abdominal tissues and organs.
• Ultrasound may guide surgeons as they perform biopsies and other procedures.
• Ultrasound may map brain structures.
• Ultrasound can visualize the interior of the ear or the eye.
• Ultrasound can find clots in veins in the leg.
• It can find bleeding in the abdomen or chest after a motor vehicle accident.

There is no special preparation for a routine ultrasound, though it may make sense to wear loose clothes for comfort. Ultrasounds are painless. Saline used in sonohysterography (a procedure in which saline is injected through the cervix into the uterus, in order to produce a more detailed ultrasound image) occasionally causes mild cramping, but over-the-counter medication is usually enough to manage this pain.

At Yale Medicine, we are skilled at using ultrasound to guide care. 

"Three percent of babies are born with major birth defects, many of which can be identified with ultrasound," says Dr. Copel. "But it's critical to have experienced eyes and hands do the scans. At Yale, we have both the experience and premium-level equipment with better technology than typically found in other settings." 

We have pioneered the use of transvaginal ultrasound early in pregnancy, as well as the use of ultrasound to diagnose pelvic pain, breast cancer and ectopic pregnancy. Yale physicians continue to explore the use of ultrasound to map the brain and track the vascular behavior of cancerous tumors.

We are also exploring the use of Doppler ultrasound to monitor cardiovascular conditions.

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NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

StatPearls [Internet].

Sonography 2nd trimester assessment, protocols, and interpretation.

Doaa Jabaz ; Suzanne M. Jenkins .

Affiliations

Last Update: November 12, 2023 .

• Continuing Education Activity

The mid-trimester obstetric ultrasound (also known as the second-trimester anatomy scan or anomaly scan) is a routine examination in many countries. It plays a pivotal role in ensuring the well-being of both the pregnant patient and the developing fetus. A cornerstone of modern prenatal care, this non-invasive imaging technique, typically performed between weeks 18 and 22 of gestation, is aimed primarily at assessing fetal anatomy and detecting any fetal anomalies. A 2-dimensional grayscale ultrasound with a curvilinear transabdominal probe is routinely used to evaluate the fetal number, viability, gestational age, anatomical survey, placental location, amniotic fluid, and maternal pelvic organs. A transvaginal probe, color Doppler, and 3-dimensional ultrasound are not routinely used. Still, they may be implemented in certain clinical scenarios, especially when placenta previa or a fetal anomaly is suspected, or to provide accurate cervical length measurement. This activity outlines the indication, safety, and technique of second-trimester ultrasound and highlights the role of the interprofessional team in conducting and interpreting the study to improve the antenatal care provided.

• Apply principles of the scanning protocol used systematically in the anatomy scan.
• Screen for common fetal abnormalities and genetic syndromes using second-trimester obstetric sonography, ensuring early detection and appropriate referral for further evaluation.
• Select appropriate transducer frequencies and imaging planes to optimize image quality and diagnostic accuracy during second-trimester obstetric sonography.
• Collaborate with an interprofessional team on strategies to optimize patient outcomes when using ultrasound for pregnancy evaluation.
• Introduction

Antenatal ultrasonography is widely used in pregnancy to assess fetal growth and anatomy. Although ultrasound screening is now an integral part of routine antenatal care, recommendations for the delivery of obstetric ultrasound vary from country to country. [1] [2]  The history of sonography in obstetrics dates from the classic 1958 Lancet paper of Ian Donald and his team from Glasgow. Clinical researchers have seized technological developments such as real-time imaging, color and power Doppler, transvaginal sonography, and 3D and 4D imaging to enhance the investigation and management of patients in areas as diverse as assessment of fetal growth and well-being, screening for fetal anomalies, and ultrasound-guided procedures as an essential component of fetal therapy. [3]

Diagnosing a fetal anomaly significantly reduces perinatal mortality and morbidity and maternal morbidity. Prenatal diagnosis enables a psychologically less traumatic and early medical termination of pregnancy. It also decreases the probable complications associated with the continuation of pregnancy and labor, prevents an unnecessary cesarean section for a fetus with lethal anomalies diagnosed too late for medical termination of pregnancy, allows for delivery to be planned at the optimal time in a well-equipped tertiary center with necessary neonatal care facilities, and allows for in utero therapy in selected cases. Performed systematically, high-resolution ultrasound can now accurately diagnose more than 200 abnormalities. A normal scan is often obtained, but there is tremendous relief of psychological distress, anxiety, and somatic symptoms after the report. [4]  The literature includes descriptions of anatomical surveys performed before 18 weeks, but other studies have repeatedly shown that more anomalies are diagnosed if the scan is performed after 18 weeks. [5] [6]

The detection rate of the screening second-trimester ultrasound scan is good in high-risk patients examined by a well-trained sonographer. On the other hand, the scan can also be sensitive in anomaly detection if a systemic searching pattern is followed in low-risk pregnant women. [7]

• Indications

An ultrasound scan performed between 18 and 22 weeks of gestation is appropriate in all pregnancies as a part of routine antenatal evaluation and screening because it provides the pregnant woman and her care provider with information about multiple aspects of her pregnancy. The obstetrical ultrasound will inform and/or confirm the number of fetuses present, the gestational age, and the location of the placenta. It will present an opportunity to diagnose congenital anomalies, detect soft markers of aneuploidy, and identify maternal pelvic pathology. [8]

• Contraindications

Based on current evidence, routine clinical scanning of every woman during pregnancy using real-time B-mode imaging is not contraindicated. The American Institute of Ultrasound in Medicine (AIUM) and the National Electrical Manufacturers Association (NEMA) regulated that the ultrasound device has to display the emitted energy through the thermal index (TI) and the mechanical index (MI). Both indexes are not perfect indicators of the risks of thermal and non-thermal bioeffects, but currently, they should be accepted as the most practical and understandable methods of estimating the potential for such risks.

Acoustic outputs in B-mode and M-mode are generally not high enough to produce deleterious effects. The significant temperature increase may be generated by spectral Doppler mode, particularly near a bone. This should not prevent using this mode when clinically indicated, provided the user has adequate knowledge of the instrument's acoustic output or access to the relevant TI. Exposure time and acoustic output should be kept to the lowest levels consistent with obtaining diagnostic information and limited to medically indicated procedures rather than purely for entertainment purposes. The "as low as reasonably achievable" (ALARA) principle best expresses this unequivocal demand for safety. [9] [10]

The examination is mainly performed with gray-scale 2D ultrasound. Harmonic imaging may enhance the visualization of subtle anatomic details, particularly in patients who scan poorly. High-frequency ultrasound transducers increase the spatial resolution but decrease the penetration of the sound beam. Several factors influence the optimal transducer and operating frequency choice, including maternal habitus, fetal position, and the approach used. Most basic examinations are satisfactorily performed with 3 to 5 MHz transabdominal transducers. [11]

The scan should be performed according to international standards and by accredited sonographers who have completed appropriate training programs supported by scientific societies. [11]

A sonologist, a physician with training and experience in this field, supervises and interprets the exams. It is the physician's responsibility to write a report based on the sonographer's data and, if necessary, personally scan the patient to validate or change the differential diagnosis. [12]

• Preparation

Reason for examination : the sonographer should validate the indication for the ultrasound examination. The last menstrual period (or the estimated delivery date) should be documented. This information is critical in correctly targeting specific structures, choosing a transvaginal and/or transabdominal technique, and determining whether additional studies may be helpful (eg, Doppler velocimetry).                                                 

Patient positioning : a semirecumbent position is most commonly used in obstetrics and gynecology. A padded table and pillow are used to ensure maximal comfort. It is preferable to elevate the head of the bed as some pregnant women cannot lie flat, especially in late pregnancy. For the transvaginal exam, a lithotomy position is used.                                                                                                                               

Special consideration for obese patients : in obese individuals, imaging can be improved by placing the transducer on the side rather than on the midline while the patient is lying on her side. The transvaginal probe is also helpful in these patients. [13]                                                                                               

Bladder filling : this is useful when the lower uterine segment is of interest. It is often of little benefit in obstetric ultrasound examination as it can falsely diagnose placenta previa or an elongated cervix. Transvaginal sonography is usually performed with an empty bladder.

• Technique or Treatment

A. Fetal Biometry and Well-being: The following sonographic parameters can help to estimate gestational age and fetal size:

• Biparietal diameter (BPD)/head circumference (HC): measured at a cross-sectional view of the fetal head at the level of the thalami and cavum septi pellucidi without visualization of the cerebellum. BPD is commonly measured from the leading edge (outer edge of the proximal skull) to the leading edge (inner edge of the distal skull), while HC is measured around the outside of the skull bone echoes. When the head shape is flattened (dolichocephaly) or round (brachycephaly), HC is more reliable than BPD.              
• Abdominal circumference (AC) or diameter: can be measured using the ellipse function at the outer surface of the skin on a transverse section of the fetal abdomen at the junction of the umbilical vein, portal sinus, and stomach.                                                                                                                             
• Femur length (FL): measured along the longest axis of the ossified diaphysis but excluding distal femoral epiphysis. [14] [15] [16]

Measurements should be performed in a standardized manner based on strict quality criteria. [17]  Reference standards appropriate for the local population should be used to interpret the measurement results. BPD and/or HC are preferable to other growth parameters in estimating gestational age. [18] [19]

B. Anatomical Survey:  This comprehensive assessment encompasses the evaluation of various fetal structures and organs to ensure their normal development and functionality. To facilitate a clear understanding of the intricacies involved in this critical diagnostic procedure, the following table offers a systematic overview of the key anatomical elements typically examined during an anatomic survey   (see  Table.  Minimal Requirements and Optional Views for the Routine Mid-Trimester Anomaly Scan). [16]

Table. Minimal Requirements and Optional Views for the Routine Mid-Trimester Anomaly Scan.

Head, including skull and brain:

• Skull: size, shape, integrity, and bone density require routine evaluation. All these characteristics can be visualized at the time of the head measurements. Size measurements are performed as mentioned in the biometry section. The skull normally has an oval shape without focal protrusions. Alterations of shape (eg, lemon, strawberry, cloverleaf) should be documented and investigated. [20] Normal skull density is manifested as a continuous echogenic structure interrupted only by cranial sutures in specific anatomical locations. The absence of this whiteness or extreme visibility of the fetal brain should raise suspicion of poor mineralization (eg, osteogenesis imperfecta, hypophosphatasia). Poor mineralization is also suggested when the skull becomes easily depressed due to manual pressure from transducer placement against the maternal abdominal wall. [21]                                                                                                
• Transventricular plane: demonstrates the anterior and posterior portion of the lateral ventricles. The frontal horns (anterior portion) appear as 2 comma-shaped fluid-filled structures separated medially by the cavum septi pellucidi (CSP), a fluid-filled cavity. The CSP becomes visible around 16 weeks and undergoes obliteration near term or in the early neonatal period. It should always be visualized between 18 and 37 weeks. The value of visualizing the CSP for identifying cerebral anomalies has been debated. However, this structure is altered with many cerebral lesions such as holoprosencephaly, agenesis of the corpus callosum, severe hydrocephaly, and septo-optic dysplasia. The posterior portion of the lateral ventricles is a complex formed by the atrium that continues posteriorly into the occipital horn. In a normal fetus, each atrium is usually measured up to 10 mm in transverse diameter; otherwise, it is generally followed to rule out early ventriculomegaly. [11] [22] [11]
• Transcerebellar plane: This plane is inferior to the BPD plane, with the probe tilted backward into the posterior fossa. The plane is correct when one can visualize the thalami and cavum septum pellucidum in the same plane as the cerebellum. The cerebellum is a dumbbell-shaped structure with symmetrical lobes. The central vermis is slightly more echogenic than the lateral lobes. The transcerebellar diameter (the widest measurement across the cerebellum perpendicular to the falx) in millimeters correlates with the gestational age up to 20 weeks and is larger than the gestational age after this time. A cerebellum measuring 2 mm less than gestational age is a concerning finding. The cisterna magna can be measured from the posterior margin of the cerebellar vermis to the inside of the occipital bone in the midline. A measurement of 2 mm to 10 mm is normal in the second and third trimesters. The nuchal fold is a measurement taken from the outer skin line to the outer bone in the midline. Less than 6 mm is considered normal up to 22 weeks. [23]
• Transthalamic plane: The anatomic landmarks include, from anterior to posterior, the frontal horns of the lateral ventricles, the CSP, the thalami, and the hippocampal gyruses. [24]

Examples of fetal head abnormalities: [25]

• Thickened nuchal skin fold: Assessment of the nuchal fold is typically made using a transcerebellar plane. A nuchal fold thickness of >6 mm is considered abnormal and is seen in 80% of newborns with Down syndrome.
• Cystic hygroma: a localized, single, or loculated, fluid-filled cavity usually occurs in the neck. It is seen in fetuses with nonimmune fetal hydrops, Turner syndrome, and trisomy syndromes 13, 18, and 21. Detection of a cystic hygroma should prompt an amniocentesis since the prevalence of cytogenetic abnormalities has been reported as high as 73%.
• Meningoceles are seen as fluid-filled cystic structures, and encephaloceles as brain-filled cystic structures that extend through a bony calvarial defect, usually in the occipital or frontal region. The entity most commonly mistaken for a small meningocele is a cystic hygroma. Careful scanning, however, will reveal no calvarial defect with a cystic hygroma. The absence of brain tissue within the sac is the single most favorable prognostic feature. Associated anomalies include Arnold-Chiari malformation, Dandy-Walker syndrome, and Meckel-Gruber syndrome (encephalocele, microcephaly, polydactyly, and cystic dysplastic kidneys). [26]

Face/neck: early detection of facial abnormalities may lead to the diagnosis of multiple syndromes, which allows counseling of the parents and optimal care of the fetus. A coronal view is usually used to assess the nose and lips. The biorbital and interorbital distances are measured by obtaining a transverse view at the level of the orbits, while a sagittal view is used to assess the facial profile and the cervical spine. [27]

Examples of face/neck abnormalities:

• Nasal bone absence/ hypoplasia: in the second trimester, hypoplastic nasal bone must be considered as important as an absence. Trisomy 21 is associated with nasal bone absence/hypoplasia in approximately 40% to 80% of cases. [28]
• Hypertelorism and hypotelorism: the distance between the 2 inner canthi, known as interocular distance (IOD), approximates the ocular diameter (specific charts are used). The distance between the 2 outer canthi is known as binocular distance (BOD). Hypertelorism (increase in IOD and BOD) may be associated with frontal encephaloceles, craniosynostoses, exposure to phenytoin, and cleft lip and palate. Hypotelorism (reduced IOD and BOD ) is usually associated with holoprosencephaly and other brain malformations. [25]
• Cleft lip and palate are the most common face malformations. Other congenital anomalies are seen in about half of the cases [25]  
• Neck masses include teratoma, lymphangioma, enlarged thyroid, branchial cleft cyst, and rarely a sarcoma. [25]

Thorax: The shape of the thorax, ribs, and homogeneous echogenicity of both lungs can be examined in axial views. The integrity of the diaphragm can be examined through the sagittal or coronal view. For the basic cardiac screening, a proper 4-chamber view of the fetal heart should be obtained with the assessment of the following: the heart rate (from 120 to 160 beats per minute), location (in the left chest), axis (about 45° toward the left side of the fetus), and size (not larger than one-third the area of the chest), 2 atria, 2 ventricles, 2 atrioventricular valves. Abnormalities like mediastinal shift, lung masses, pleural effusions, diaphragmatic hernia, tachycardia, dextrocardia, cardiomegaly, pericardial effusion, ventricular septal defect, and abnormal chamber size can be detected. Cardiomegaly can be due to congenital heart disease, severe fetal anemia, or a small chest. [16]

Abdomen: organ situs should be determined. The fetal stomach should be identified in its normal position on the left side. The fetal umbilical cord insertion site should be examined for evidence of a ventral wall defect. Normal kidneys, along with the adrenal glands, are seen on either side of the spine just below the level of the fetal stomach. [21] The kidneys appear echogenic in the early weeks and gradually become hypoechoic compared with the adjacent bowel and liver. Fetal ureters are not usually visible antenatally unless they are dilated. Persistent absence of the bladder should be considered abnormal from 15 weeks. [29]

Examples of fetal abdomen abnormalities:

• Echogenic bowel: defined as bowel with an echogenicity ≥ that of surrounding bone; the differential diagnosis for this finding is broad and includes normal variant, congenital viral infection, cystic fibrosis, aneuploidy, and intra-amniotic bleeding. Previous reports have also suggested an increased incidence of intrauterine growth restriction (IUGR) and intrauterine fetal demise in fetuses with echogenic bowel. [30]
• Fetal bowel dilatation: characterized by fluid-filled intestinal loops that measure at least 15 mm in length or 7 mm in diameter. Dilated fetal bowel is a sign of intestinal mechanical or functional obstruction. Its prevalence will depend on the underlying condition: bowel atresia or stenosis, malrotation with volvulus, meconium ileus, total colonic aganglionosis, and meconium plug syndrome. [31]
• Dysplastic kidneys: result from abnormal development of the glomeruli and nephrons along with a disproportionately increased stroma. They appear large and bright, usually with cystic spaces. Typically, the cysts are multiple, thin-walled with no connections, randomly placed in the renal parenchyma, and form an irregularly outlined kidney. Rarely, dysplastic kidneys appear uniformly echogenic on ultrasound without cysts and can be difficult to differentiate from normal kidneys. The renal pelvis and ureters are not seen. Dysplastic kidneys are often associated with other syndromes such as VACTERL association, Meckel–Gruber syndrome, Fraser syndrome, and CHARGE syndrome. Most babies with isolated unilateral multicystic renal disease tend to have a good outcome. The size and number of cysts in unilateral multicystic disease do not influence the outcome. Bilateral involvement is generally associated with severe oligohydramnios and has a poor prognosis owing to the resultant pulmonary hypoplasia. [29]
• Infantile polycystic kidney disease (Potter type I): has an autosomal recessive inheritance. It carries a 25% risk in subsequent pregnancies. The age of onset of this condition is varied and is subdivided into perinatal, neonatal, infantile, and juvenile. The perinatal onset type is the most common and characterized by bilaterally enlarged and homogenously hyperechogenic kidneys, with or without oligohydramnios, renal failure occurring in utero, and 40% to 50% affected with hepatic fibrosis. Kidneys are affected earlier, and hepatic fibrosis is common in late-onset polycystic kidney disease. Perinatal mortality is usually caused by pulmonary hypoplasia after severe oligo- or anhydramnios. [29]
• When evaluating urinary bladder distention and pyelectasis in the male fetus, severe or progressive oligohydramnios, progressive bladder wall thickening, and a persistently dilated posterior urethra are most consistent with posterior urethral valves. Although oligohydramnios, dilated posterior urethra, and bladder wall thickening can be seen in prune belly syndrome, these findings are likely transient. Vesicoureteral reflux, ureterovesical junction obstruction, and non-refluxing, nonobstructive megacystis–megaureter should be considered when pyelectasis and megacystis are present without additional bladder, urethral, or renal abnormalities. [32]
• The 2 most common congenital abdominal wall defects are omphalocele and gastroschisis. Omphalocele appears as an outpouching of the abdominal wall recovered by an inner membrane of the parietal peritoneum and an outer layer of amnion and Wharton jelly. It usually occurs at the base of the umbilical cord, the cord insertion being located at the apex of the herniated sac, which contains variable amounts of different visceral organs, usually, bowel or liver or both; concurrent malformations are described in up to 74% of fetuses. It is considered the consequence of a failure of bowel loops to return to the body cavity after their normal physiological herniation into the umbilical cord during the sixth to tenth week of development. Gastroschisis is characterized by a full-thickness abdominal wall closure defect, which permits evisceration of the fetal abdominal contents, specifically bowel herniation. The orifice of the defect is usually small and, nearly always, is right-sided to umbilicus insertion. There is no surrounding membrane or sac, and these free-floating loops of the intestine may become edematous due to direct exposure to the amniotic fluid. Children of young mothers are more susceptible. It is not usually associated with other anomalies, but sometimes it may occur combined with congenital heart disease, ectopia, cordis, neural tube, and diaphragmatic defects. Survival rates are good (85% to 97%). [33]

Spine: The choice of the scanning planes used to evaluate the spine's integrity depends upon the fetal position. However, a longitudinal view should always be obtained because it may reveal other spinal malformations, including vertebral abnormalities and sacral agenesis. In sagittal planes, the ossification centers of the vertebral body and posterior arches form 2 parallel lines that converge in the sacrum. In the second and third trimesters of gestation, the conus medullaris is usually found at the level of L2 to L3. The integrity of the neural canal is inferred by the regular disposition of the ossification centers of the spine and the presence of soft tissue covering the spine. [11]

Extremities: assessment of all limbs, including the hands and feet, should be done to evaluate the bone shape, size, and integrity. Most prenatal-onset skeletal dysplasias present with a relative disproportion of the skeletal measurements compared with those of the cranium. One of the most critical determinations that ultrasound must make is neonatal or infantile lethality. Lethality occurs in most skeletal dysplasias due to a small chest circumference and resultant pulmonary hypoplasia. Using ultrasound criteria for lethality, a chest-to-abdominal circumference ratio of <0.6 and a femur length-to-abdominal circumference ratio of <0.16 strongly suggest lethality. [34]

Examples of fetal skeletal dysplasia:

• Thanatophoric Dysplasia: is the most common lethal skeletal dysplasia. Inheritance is generally autosomal dominant. It is characterized by disproportionate dwarfism with very short extremities, which are bowed in type 1 and maybe straight in type 2. The trunk length is normal, but the thorax is narrow. There is a distinct flattening of vertebral ossification centers (platyspondyly) and a large head, depressed nasal bridge, prominent forehead, and protruding eyes. Skull deformity is often present due to the premature closure of cranial sutures. Cloverleaf skull deformity is generally seen in type 2. Polyhydramnios is present in almost 50% of cases. [35]
• Achondroplasia: is the most common nonlethal skeletal dysplasia without mental impairment. At about 20 weeks of gestation, fetuses with achondroplasia had normal biometric parameters, including FL, which became abnormally short only in the third trimester. The micromelia is rhizomelic, and the head tends to be large. Typical facial features include a prominent forehead, depressed nasal bridge, and mid-face hypoplasia. The phalanges are short; typical gaps are seen between the fingers, and digital deviation leads to the appearance of a " trident'’ hand. [36]  
• Osteogenesis Imperfe ista: a heterogeneous group of disorders caused by gene mutations that encode type I collagen, increasing bone fragility. The major features are collapsed vertebral bodies, rib fractures, and, in more severe cases, thin shafts with fractures and bowing deformities. Fetal movements may be reduced. The skull may be thinner than usual, and the weight of the US probe may deform the head quite easily. [35]

Gender: If the external genitalia is to be visualized for gender determination, imaging in both mid-sagittal and axial planes is recommended to minimize error. A mid-sagittal image of the lower abdomen below the cord insertion demonstrates the penis and scrotum caudally to the cord insertion in a male fetus and the flat mons pubis caudal to the cord insertion in a female fetus. A transverse image, just below the level of the bladder, best taken with the knees separated, demonstrates the penis and scrotum between the thighs in males and 3 lines representing the labia in females. Testes are undescended at this gestation and usually descend later in the third trimester. [23]

C. Placenta and Amniotic Fluid Volume: assessing the placental location, appearance, and its relation with the internal cervical os is essential. Amniotic fluid volume can be estimated subjectively or by measuring the deepest vertical pocket. A vertical pocket of ≤2 cm and ≥8 cm are regarded as oligohydramnios and polyhydramnios, respectively. [16] [37] [16]

D. Cervix, Uterine Morphology, and Adnexa: There is insufficient evidence to recommend routine screening of asymptomatic and symptomatic pregnant women with transvaginal ultrasound cervical length. [38]   Maternal adnexal or uterine pathologies like masses that may interfere with fetal growth or labor should be documented.

• Clinical Significance

The mid-trimester ultrasound scan should be performed according to international standards and preferably by accredited sonographers.

When a structural abnormality is detected, the obstetrician wants to know: How severe is the anomaly? Is it lethal? Does the fetus have a chromosomal abnormality? What is the cause? How can the fetus be best salvaged? A fetal karyotype is indicated when there are multiple structural malformations, associated fetal growth retardation, certain anomalies like cystic hygroma, non-immune hydrops, and cardiovascular defects. It is better to know the fetal karyotype during pregnancy rather than wait until after delivery, as the baby may not often survive, the chromosomal cultures might fail, and valuable information might be lost, which could be vital for future counseling.

Management guidelines for a lethal anomaly like anencephaly and bilateral renal agenesis indicate termination of the pregnancy. If the defect is nonprogressive and correctable at birth, as in mild hydronephrosis with good renal function, the decision would be to wait until term delivery followed by neonatal treatment. Fetal therapy of a structurally malformed fetus requires a multidisciplinary team comprised of the obstetrician, geneticist, neonatologist, and pediatric surgeon. Malformations that require correction in utero are those that would cause deterioration of the fetal organs if left alone. [4]

• Enhancing Healthcare Team Outcomes

The optimal obstetric outcome can be expected by the early initiation of prenatal care. An essential element of prenatal care is the early detection of possible malformations, which is the primary goal of the mid-trimester anomaly scan. This cannot be achieved without optimized coordination among interprofessional team members, including well-trained sonographers, specialists-referral and parent counseling systems, nurses, obstetricians, fetal medicine specialists, surgeons, and others.

Health professionals involved in second-trimester obstetric sonography must possess specific technical skills, including proficiency in operating ultrasound equipment, identifying fetal structures, and interpreting sonographic images accurately. Continuous training and skill development are essential to maintain competency in this rapidly evolving field.

A well-defined strategy involves establishing standardized protocols and guidelines for conducting second-trimester obstetric sonography. This ensures consistency in care delivery and reduces variations in practice. It also includes setting objectives for the scan, such as screening for fetal anomalies or assessing fetal growth based on the patient's specific clinical indications.

Effective interprofessional communication is vital for the success of obstetric sonography. Physicians, nurses, and sonographers must collaborate closely to ensure that the scan is performed correctly, that findings are communicated accurately, and that patients receive clear explanations. Open and respectful communication among team members fosters trust and enhances patient care.

Ultimately, the interprofessional team can enhance patient-centered care in second-trimester obstetric sonography by fostering a collaborative approach that emphasizes skills development, shared responsibilities, effective communication, and care coordination. This leads to improved patient outcomes, safety, and team performance while respecting the unique challenges and complexities of obstetric care.

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Disclosure: Doaa Jabaz declares no relevant financial relationships with ineligible companies.

Disclosure: Suzanne Jenkins declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Jabaz D, Jenkins SM. Sonography 2nd Trimester Assessment, Protocols, and Interpretation. [Updated 2023 Nov 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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• Review Routine ultrasound for fetal assessment before 24 weeks' gestation. [Cochrane Database Syst Rev. 2021] Review Routine ultrasound for fetal assessment before 24 weeks' gestation. Kaelin Agten A, Xia J, Servante JA, Thornton JG, Jones NW. Cochrane Database Syst Rev. 2021 Aug 26; 8(8):CD014698. Epub 2021 Aug 26.
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Fetal Ultrasound

What is fetal ultrasound.

Fetal ultrasound is a test used during pregnancy. It creates an image of the baby in the mother's womb (uterus). It’s a safe way to check the health of an unborn baby. During a fetal ultrasound, the baby’s heart, head, and spine are evaluated, along with other parts of the baby. The test may be done either on the mother’s abdomen (transabdominal) or in the vagina (transvaginal).

There are several types of fetal ultrasound:

• Standard ultrasound. The test uses sound waves to create two-dimensional images on a computer screen.
• Doppler ultrasound. This test shows the movement of blood through the umbilical cord, in the baby’s heart, or between the baby and the placenta.
• 3-D ultrasound. This test shows a lifelike image of an unborn baby.

Ultrasound uses an electronic wand called a transducer to send and receive sound waves. No radiation is used during the procedure. The transducer is moved over the abdomen, and sound waves move through the skin, muscle, bone, and fluids at different speeds. The sound waves bounce off the baby like an echo and return to the transducer. The transducer converts the sound waves into an electronic image on a computer screen.

Why might I need fetal ultrasound?

Fetal ultrasound is a routine part of prenatal care in the U.S. This is because it’s a low risk procedure that gives important information. A routine prenatal ultrasound can check for defects or other problems in the fetus. The following can be examined:

• Abdomen and stomach
• Arms, legs, and other body parts
• Back of the neck
• Head and brain
• Heart chambers and valves
• Placenta placement
• Umbilical cord
• Urinary bladder

A fetal ultrasound can also show:

• If a woman is pregnant with multiple babies
• The gestational age of a baby
• Where to place the needle during removal of amniotic fluid (amniocentesis)
• Whether a feus is growing properly

Your healthcare provider may have other reasons to request a fetal ultrasound.

What are the risks of fetal ultrasound?

All procedures have some risks. The risks of this procedure include:

• Mild discomfort from the transducer on the abdomen or in the vagina
• Reaction to a latex covering for the transducer, if you have a latex allergy

In some cases, an ultrasound may appear to show a problem that is not there called false-positive. The test can also miss a problem that is there called false-negative. In some cases, additional testing may be needed after a fetal ultrasound.

Fetal ultrasound is sometimes offered in nonmedical settings. This is done as a way to give keepsake images or videos for parents. In these cases, it’s possible for untrained staff to misread the images and give parents incorrect information. Make sure to have fetal ultrasound done by trained medical staff. Talk with your healthcare provider if you have questions.

Your risks may vary depending on your general health and other factors. Ask your healthcare provider which risks apply most to you. Discuss any concerns you may have.

How do I get ready for fetal ultrasound?

Your healthcare provider will explain the procedure to you. Ask any questions you may have. You may be asked to sign a consent form that gives permission to do the procedure. Read the form carefully. Ask questions if anything is not clear.

Tell your healthcare provider if you:

• Are sensitive to or allergic to any medicines, latex, tape, or anesthetic medicines (local and general)
• Take any medicines, including prescriptions, over-the-counter medicines, vitamins, and herbal supplements

You may be asked to drink several glasses of water before the procedure. This can help give clearer images.

What happens during a fetal ultrasound?

You may have your procedure as an outpatient. This means you can go home the same day. Or it may be done as part of a longer stay in the hospital. The way the procedure is done may vary. It depends on your condition and your healthcare provider’s methods. In most cases, the procedure will follow this process:

• If you are having a transabdominal ultrasound, you will be asked to raise your shirt to expose your abdomen. For a transvaginal ultrasound, you will be asked to remove your clothes from the waist down. You will be given a sheet to place over your legs. You may be asked to remove jewelry or other objects.
• You will lie on an exam table, either on your back or on your side.
• For a transabdominal procedure, a clear gel will be placed on the skin of your belly (abdomen). The transducer will be moved over the area and pressed against the skin.
• For a transvaginal ultrasound, you will need to lie on your back. Your feet will be in stirrups and your knees apart.
• A vaginal transducer probe will be covered with a sterile cover. A lubricant will be put on the probe. The probe will be inserted into the vagina. The technician will move the probe around to capture a series of images. The probe will then be removed.

What happens after fetal ultrasound?

You will be given tissue to wipe off excess gel. You can go home shortly after the test. Your healthcare provider will talk with you about the results. You may get other instructions after the procedure.

Before you agree to the test or the procedure make sure you know:

• The name of the test or procedure
• The reason you are having the test or procedure
• What results to expect and what they mean
• The risks and benefits of the test or procedure
• What the possible side effects or complications are
• When and where you are to have the test or procedure
• Who will do the test or procedure and what that person’s qualifications are
• What would happen if you did not have the test or procedure
• Any alternative tests or procedures to think about
• When and how will you get the results
• Who to call after the test or procedure if you have questions or problems
• How much will you have to pay for the test or procedure

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Fetal Ultrasound

What is fetal ultrasound?

Fetal ultrasound is a test used during pregnancy. It creates an image of the baby in the mother's womb (uterus). It’s a safe way to check the health of an unborn baby. During a fetal ultrasound, the baby’s heart, head, and spine are evaluated, along with other parts of the baby. The test may be done either on the mother’s abdomen (transabdominal) or in the vagina (transvaginal).

There are several types of fetal ultrasound:

Standard ultrasound.  The test uses sound waves to create 2-D images on a computer screen.

Doppler ultrasound.  This test shows and measures the movement of blood through the uterus, umbilical cord, in the baby’s heart, or around the baby's body.

3-D ultrasound.  This test shows a lifelike image of an unborn baby.

Ultrasound uses an electronic wand called a transducer to send and receive sound waves. No radiation is used during the procedure. The transducer is moved over the abdomen, and sound waves move through the skin, muscle, bone, and fluids at different speeds. The sound waves bounce off the baby like an echo and return to the transducer. The transducer converts the sound waves into an electronic image on a computer screen.

Why might I need fetal ultrasound?

Fetal ultrasound is a routine part of prenatal care in the U.S. This is because it’s a low risk procedure that gives important information. A routine prenatal ultrasound can check for defects or other problems in the unborn baby. The following can be examined: 

Abdomen and stomach

Arms, legs, and other body parts

Back of the neck

Head and brain

Heart chambers and valves

Placenta placement

Umbilical cord

Urinary bladder

A fetal ultrasound can also show:

If a woman is pregnant with multiple babies

The gestational age of a baby

Where to place the needle during removal of amniotic fluid (amniocentesis)

Whether a baby is growing normally

Your healthcare provider may have other reasons to request a fetal ultrasound.

What are the risks of fetal ultrasound?

All procedures have some risks. The risks of this procedure include:

Mild discomfort from the transducer on the abdomen or in the vagina

Reaction to a latex covering for the transducer, if you have a latex allergy

In some cases, an ultrasound may appear to show a problem that is not there called false-positive. The test can also miss a problem that is there called false-negative. In some cases, additional testing may be needed after a fetal ultrasound.

Fetal ultrasound is sometimes offered in nonmedical settings. This is done as a way to give keepsake images or videos for parents. In these cases, it’s possible for untrained staff to misread the images and give parents incorrect information. Make sure to have fetal ultrasound done by trained medical staff. Talk with your healthcare provider if you have questions.

Your risks may vary depending on your general health and other factors. Ask your healthcare provider which risks apply most to you. Discuss any concerns you may have.

How do I get ready for fetal ultrasound?

Your healthcare provider will explain the procedure to you. Ask any questions you may have. You may be asked to sign a consent form that gives permission to do the procedure. Read the form carefully. Ask questions if anything is not clear.

Tell your healthcare provider if you:

Are sensitive to or allergic to any medicines, latex, tape, or anesthetic medicines (local and general)

Take any medicines, including prescriptions, over-the-counter medicines, vitamins, and herbal supplements

You may be asked to drink several glasses of water before the procedure. This can help give clearer images.

What happens during a fetal ultrasound?

You may have your procedure as an outpatient. This means you can go home the same day. Or it may be done as part of a longer stay in the hospital. The way the procedure is done may vary. It depends on your condition and your healthcare provider’s methods. In most cases, the procedure will follow this process:

If you are having a transabdominal ultrasound, you will be asked to raise your shirt to expose your abdomen. For a transvaginal ultrasound, you will be asked to remove your clothes from the waist down. You will be given a sheet to place over your legs.  You may be asked to remove jewelry or other objects.

You will lie on an exam table, either on your back or on your side.

For a transabdominal procedure, a clear gel will be placed on the skin of your belly (abdomen). The transducer will be moved over the area and pressed against the skin.

For a transvaginal ultrasound, you will need to lie on your back. Your feet will be in stirrups and your knees apart.

A vaginal transducer probe will be covered with a sterile cover. A lubricant will be put on the probe. The probe will be inserted into the vagina. The technician will move the probe around to capture a series of images. The probe will then be removed.

What happens after fetal ultrasound?

You will be given tissue to wipe off excess gel. You can go home shortly after the test. Your healthcare provider will talk with you about the results. You may get other instructions after the procedure.

Before you agree to the test or the procedure make sure you know:

The name of the test or procedure

The reason you are having the test or procedure

What results to expect and what they mean

The risks and benefits of the test or procedure

What the possible side effects or complications are

When and where you are to have the test or procedure

Who will do the test or procedure and what that person’s qualifications are

What would happen if you did not have the test or procedure

Any alternative tests or procedures to think about

When and how you will get the results

Who to call after the test or procedure if you have questions or problems

How much you will have to pay for the test or procedure

Related Links

• Johnson Center for Pregnancy and Newborn Services
• Fetal and Pregnancy Health
• Placental Disorders
• Fetal Echocardiography
• Ultrasound in Pregnancy
• Biophysical Profile

Related Topics

Maternal and Fetal Testing Overview

Sickle Cell Disease and Pregnancy

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• Data Descriptor
• Open access
• Published: 24 August 2024

A benchmark for 2D foetal brain ultrasound analysis

• Mariano Cabezas   ORCID: orcid.org/0000-0002-4417-1704 1   na1 ,
• Yago Diez 2   na1 ,
• Clara Martinez-Diago 3   na1 &
• Anna Maroto 3   na1  

Scientific Data volume  11 , Article number:  923 ( 2024 ) Cite this article

Metrics details

• Scientific data

Brain development involves a sequence of structural changes from early stages of the embryo until several months after birth. Currently, ultrasound is the established technique for screening due to its ability to acquire dynamic images in real-time without radiation and to its cost-efficiency. However, identifying abnormalities remains challenging due to the difficulty in interpreting foetal brain images. In this work we present a set of 104 2D foetal brain ultrasound images acquired during the 20th week of gestation that have been co-registered to a common space from a rough skull segmentation. The images are provided both on the original space and template space centred on the ellipses of all the subjects. Furthermore, the images have been annotated to highlight landmark points from structures of interest to analyse brain development. Both the final atlas template with probabilistic maps and the original images can be used to develop new segmentation techniques, test registration approaches for foetal brain ultrasound, extend our work to longitudinal datasets and to detect anomalies in new images.

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Background & summary.

Foetal brain development is a complex sequence of events ocurrying throughout gestation. From early stages of embryonic development, the brain undergoes structural changes until several months after birth 1 , 2 . Therefore, understanding normal brain development is essential to identify potential deviations which may lead to neurological disability. Specifically, various studies have described normal milestones within a specific chronology 3 , 4 , 5 . These changes can be observed by experts in prenatal diagnosis with skills in foetal neurology using ultrasound (US) and magnetic resonance imaging (MRI). However, identifying neurodevelopmental deviations is challenging due to the difficulty in image interpretation 5 .

Currently, US is the established technique for screening due to its ability to acquire dynamic images in real-time without radiation and cost-efficiently. The international guidelines recommend the acquisition of a routine first trimester US within the 11th - 14th weeks and a mid-trimester scan within the 19th - 22th weeks of gestation for anatomical evaluation of the foetus 6 . The standard foetal assessment includes the evaluation of planes acquired using 2D-US. Even though 3D-US is considered useful for prenatal diagnosis of some disorders (mainly involving the face, the skeleton, the cardiovascular system or the brain), the major obstacles for 3D-US implementation worldwide as the main routine acquisition type are related to foetal motion artefacts and acoustic shadowing during volume acquisition. Another widely used prenatal imaging technique is MRI. However, MRI is not used as a primary screening tool and is instead used as a complementary acquisition when foetal abnormalities are suspected (even in selected high-risk cases) 7 , 8 . Moreover, foetal MRI performed before the 18th-22nd weeks does not usually provide additional information to that obtained by US. Generally, MRI provides a detailed visualization of structures between the 26th and 32nd weeks, being superior to US and less susceptible to limitations from maternal body conditions and foetal presentation (bones do not produce occlusion artefacts in MRI) 8 . Within this context, a foetal brain US atlas based on 3D scans of healthy foetuses was recently published 9 . This atlas demonstrates the feasibility of assessing structural changes in the cortex and in the subcortical grey matter by US. Furthermore, this chronological US-based atlas of the foetal brain at each gestational age has the potential to become a useful tool to detect development abnormalities when combined with the standard planes of mid-trimester routine scans. However, considering that the primary diagnostic tool in pregnancy remains 2D-US, studies aimed at improving the identification of fetal structures should focus on 2D-US.

As there is a degree of variability in the shapes of anatomical regions between individuals, atlases are typically built by taking into account a number of images considered to fall within normal parameters. A crucial step in this process is finding corresponding regions between different images and warping them to a common space, a process known as image registration. In the case of US atlases, the lower signal-to-noise ratio (SNR), the differences in location for the foetuses due to the lack of a robust localization technique and the presence of image artefacts that may blur or shadow salient features and structure boundaries makes this process particularly challenging.

In this paper we present a set of foetal brain 2D-US images with manual landmark annotations of structures of interest and soft probabilistic maps for those structures based on these landmarks in a common space. The aim of this dataset is to provide a set of US images as a starting point to study registration and segmentation of foetal brain scans and provide tools for researchers focusing on foetal brain development. Furthermore, in combination with the recently published 4D atlas, this dataset can be also used to study 2D to 3D US registration and techniques to determine the “real age” of gestation and potential abnormalities when comparing to the atlas. To define a common space, a registration algorithm based on fitting an ellipse to the skull segmentation of a convolutional neural network (CNN) was used to align all the images to a common space. In order to avoid misalignements caused by image quality, a subject ellipse was used to estimate an affine transformation to a common (atlas) space. We also took advantage of the automatic differentiation capabilities of the pytorch package in three fundamentally different optimization settings (training a CNN for segmentation, fitting an ellipse to a segmentation boundary, and estimating an affine transformation).

In order to create the dataset presented, the data collected was processed as follows: First, the skull was automatically segmented using a UNet network, then an ellipse was automatically fitted to the shape of the skull. These ellipses where used to register each image to a reference image. Soft probabilistic maps where built using the set of registered images. The rest of this section includes details on each of these steps and a summary of other publicly available datasets. A visual summary can be found in Fig.  1 .

Methodology: 1) The skull is automatically segmented using a UNet network (top-left). 2) An ellipse is fitted to the skull segmentation (top-right) 3) to estimate an affine transformation to a reference image (bottom-right). The axes of the fitted ellipse are warped to the image coordinate axes and are re-scaled to fit the ellipse in the reference image (bottom-left).

Ultrasound data

A prospective cohort of low-risk pregnant women was recruited at routine mid-trimester foetal ultrasound scan. All participants initiated antenatal care before the 12th weeks of gestation, underwent the first trimester ultrasound scan between 12th and 14th weeks and had a low risk for aneuploidies in the first trimester combined screening. Written informed consents were obtained from participants. A private dataset of 70 pregnant women with a routine mid-trimester foetal ultrasound scan at [20 ~ 20.6] weeks without detected abnormalities was acquired, totalling 104 scans (8 women were scanned three times, 18 women were scanned twice and the remaining 44 women were only scanned once). The median of the maternal age was 31 (range 18-42). Images were acquired using high-frequency transabdominal probe (C2-9) of Voluson E10 ultrasound system. For each subject, a transverse view of the foetal head demonstrating a standard normative transcerebellar scanning plane was manually annotated by2 trained clinical experts (Fig.  1 top right). Both experts were part of the Prenatal Diagnosis Unit, one with over 10 years of experience and the other with 5 years of experience. The experts jointly participated in the annotations of each imageto highlight common structures related to brain development as follows:

Skull : 4 landmarks from the inner line of the skull were annotated: 2 at the level of the middle line and the other 2 in a perpendicular imagined line at the level of the posterior corners of the cavum septi pellucidi (CSP).

Cerebellar peduncles (Thalami) : 1 landmark marking the edges of both thalami at the middle line and the outer edges of the concavity shape were annotated (3 total points).

Cerebellum : 8 landmarks on the perimeter the cerebellum were annotated. Specifically, 2 points from the midline, 2 points from the cerebellum external edges and 4 points in the middle of each cerebellum hemisphere.

Cavum septi pellucidi (Cavum) : 4 points, each marking one corner of its rectangular shape, were annotated.

Sylvian Fissure (Sylvius) : 2 landmarks, one for each sylvian fissure edge, and 1 landmark in the inflection point of the fissure were annotated. For all the images, only the inferior fissure was visible.

Midline : 1 landmark in the upper edge of the midline and 1 landmark in the upper edge of the CSP were annotated.

An important aspect of acquisition is that the sylvian fissure was always scanned on the lower part of the image, irrespective of the head orientation (left to right or right to left). This phenomena has important implications for registration. When aligning all images to a common 3D space (for example a 3D atlas template) in order to have all images facing the same direction, the transformation can be modeled with 180° rotations over the y axis. If the transformation is limited to the 2D space (image coordinates), a mirroring operation has to be applied to align all images. In the code we provide to process the images, the second option is used for simplicity. Furthermore, our common space is oriented from left to right (anterior to posterior).

Data acquisition was approved by the ethics committee “Comitè d’Ètica d’Investigació amb Medicaments CEIM GIRONA” with the code 2023.067. Furthermore, the subjects were informed and consented to the open publication of the data.

Skull segmentation database

The segmentation dataset used to train a skull segmentation CNN was downloaded from the HC18 grand challenge on “automated measurement of foetal head circumference using 2D ultrasound images” 10 , 11 . The challenge comprises a set of 800 × 540-pixel 2D US images with a pixel sizes ∈ (0.052, 0.326) mm. The data set was split into 999 images for training and 335 for testing. For each image in the training set, an ellipse was manually fitted to the HC by a trained sonographer but precise segmentations of the true skull boundary were not provided. No pre-processing techniques were applied to the images apart from computing the z-score of the intensity values with respect to the mean and standard deviation of the whole image (non-0 intensities) before feeding them to the segmentation network. The network trained with all the training set images was then used to roughly segment the skull of our own 2D US images and provide a starting point to fit an ellipse.

Other public databases

To the best of our knowledge, only 3 other large datasets focusing on 2-D ultrasound have been made publicly available, including HC18. As mentioned on our brief description of HC18, the annotations are limited (only an ellipse roughly representing the skull is given), the subjects present a large distribution of gestational ages and no further information on the acquired plan e is provided. For studies focusing on the segmentation of structures of interest or registration, new annotations would be needed. Furthermore, the gestational age and acquisition plane for each subject can have a large impact on the appearance of the structures. Another public dataset presented by Burgos-Artizzu et al . 12 includes a large set of images (12400) ranging from the 18th to the 40th week of pregnancy. Images were acquired using six different machines and labelled at the image level. Similarly to HC18, the dataset presents a large range of gestational ages and limited annotations that could only be used to develop classification algorithms. Finally, Alzubaidi et al . 13 released a public dataset of 3832 high resolution images. In contrast with the other two datasets, no mention of gestational age is provided and once again rough annotations in the form of bounding boxes are provided. Moreover, the authors also highlight image resampling as an additional shortcoming of their dataset.

In our dataset, we focus on a specific acquisition plane and gestational age as defined by international guidelines and raw images are provided. Furthermore, landmarks for the most salient points of each structure of interest are provided with software tools to estimate finer-grained masks and bounding boxes around the landmarks. In that sense, our dataset provides a useful tool to address multiple image analysis problems including registration that could not be easily approached with other available datasets and no additional annotations.

Registration method

Medical image registration 14 , 15 is a necessary initial step for many medical image processing applications that rely on group-wise analysis. Typically, images are registered in pairs: one of them is defined as the “fixed” image (or reference) and the other as the “moving” image. The moving image is then warped using a transformation function to generate the final “moved” image. The transformation is commonly optimised using a predefined metric that computes image similarity between the reference and the moved image after transformation. Here, we present a coarse registration method to roughly align different foetal ultrasound images as described in the following sections and illustrated in Fig.  1 . We chose one of the images as the common “reference” image (image 10) and registered the remaining images to it.

Automatic Skull segmentation using a Unet

The Unet architecture 16 is one of the most common CNN architectures for image segmentation. Due to its encoder-decoder structure and the use of skip connections, precise segmentations based on multi-scale features can be obtained for a variety of segmentation applications 17 . Consequently, we used a 2D Unet trained on the HC18 challenge dataset to provide a rough segmentation of the skull for all the images in our registration dataset (see Fig.  1 for an example). Specifically, the encoder and decoder blocks are comprised of 6 convolutional layers with a residual connection 18 of 32, 32, 128, 256, 256 and 1024 features each (inverse order for the decoder) and a bottleneck of 1024 features. To optimise the weights of the Unet, the Adam algorithm with default initial learning rate was used to minimise the binary cross-entropy loss.

Ellipse Registration

An ellipse is a planar curve representing the locus of the points with constant added distances to two “focal points”, as expressed by the quadratic equation:

With points that satisfy f E ( X ,  Y ) ≠ 0 being inside the ellipse perimeter ( f E ( X ,  Y ) < 0) or outside of it ( f E ( X ,  Y ) > 0). The general equation’s coefficients can be obtained from known semi-major axis a (represented by the magnitude of the turquoise vector in Fig.  1 ), semi-minor axis b (represented by the magnitude of the cyan vector in Fig.  1 ), centre coordinates ( x 0 ,  y 0 ) (represented by the point where the two vectors meet in Fig.  1 ) and rotation angle θ (the angle from the positive horizontal axis to the ellipse’s major axis as observed in Fig.  1 ) using the formulae:

These expressions can be derived from the canonical equation \(\frac{{x}^{2}}{{a}^{2}}+\frac{{y}^{2}}{{b}^{2}}=1\) by an affine transformation of the coordinates ( x ,  y ) (with a translation (− x 0 , − y 0 ) and an angle θ ).

Given the set of pixel coordinates of the skull segmentation ( \(({X}_{skull},{Y}_{skull})=\,[({x}_{1},{y}_{1}),\ldots ,({x}_{N},{y}_{N})]\) ) we can fit an ellipse using its parameters ( a , b , x 0 , y 0 and t h e t a ) by minimising the following objective function:

where the coefficients A to F are substituted in f E by their definitions in equations ( 2 ) to ( 7 ) and the estimated ellipse parameters.

This process is repeated 5 times, removing erroneous points of the skull segmentation mask that are one standard deviation away from the mean ellipse error according to Eq. ( 10 ). With the parameters of the skull ellipse estimated through optimisation, we can now define an affine transformation (referred to as Ellipse from now on) to move the brain to the centre of the image as:

Affine image registration

For comparison, a regular rigid registration of 6 unrestricted parameters was performed with different initialisations. From the most basic identity initialisation (referred to as Affine ), to an initialisation using the ellipse parameters of the reference image (referred to as Affine (Reference ellipse) ) and a refinement of the ellipse-based affine transformation from Eq. ( 11 ) (referred to as Ellipse + Affine ).

Probabilistic maps

Once the images are co-registered to a common space based on their ellipse, a two-step process is performed to generate probabilistic maps for all the structures with more than 2 landmarks.

First, the concave hull of each structure is computed using the alphashapes package. This concave hull represents a rough segmentation of the location of the structure. Second, the segmentations for all the subjects are averaged per structure to determine the probability of each pixel to belong to that structure.

This approach has some limitations. Namely, some structures have a polygonal shape, even though they are actually curves (e.g. sylvius) and the final masks are only a rough representation of the real boundaries (e.g. cerebellum). However, these polygonal maps can be still used to determine growth milestones and to provide a rough location of the structure of interest.

Data Records

The original images with manual landmark annotations (Gimp image editing tool format) and the co-registered images and probabilistic maps used within this paper can be found on figshare 19 , and are organised with subject id number (1-52) and an additional number for multiple scans (e.g. 36 and 36.1). Co-registered images are saved in JPEG format with the “_registered” suffix. The final probability maps estimated using a combination of co-registered landmarks and the alphashape package are provided as JPEG images with the name of the structure (e.g. sylvius.jpeg), while comma separated value (CSV) files with the point landmarks of all registered subjects are compressed into a single zip file.

In total, the released dataset consists of 104 annotated 2D US images of foetal brains on the 20th week of pregnancy. The manual annotations are described in Ultrasound dataset section.

Technical Validation

To validate the techniques used for co-registration to a common space, we focused on common medical imaging metrics for registration using landmarks. In this section we describe these metrics and provide some qualitative and quantitative results of the alignment (including a visualisation of the probabilistic maps of one of the structures of interest).

Regarding the quality of the US images, all images were acquired with a high frequency ultrasound probe (2-9 MHz) that provides high resolution images following the ISUOG recommendations. The most constraining factor for the quality of the ultrasound images is maternal obesity. However, for this study we excluded women with maternal morbid obesity (body mass index > 40) as it is one of the factors for high-risk pregnancy. To further illustrate that point, we provide a comparative example between an image from the dataset and a lower quality one in Fig.  2 .

Qualitative comparison between a low quality image where structures are not clearly visible and and image from the dataset.

Evaluation metrics

For this study we chose to use the anatomical knowledge defined by expert annotations as the main directive to evaluate the quality of the registration and avoid focusing exclusively on common pixel-metrics that might be unreliable and disconnected from physical properties 20 . Specifically, we used the point annotations described in the Ultrasound Data section with two point-based metrics and one area-based metric for every anatomical structure with more than 2 points (all the structures except the midline). For completeness sake, we also included the structural similarity index metric (SSIM) pixel-based metric.

Point-based metrics

In order to penalise partial matches between anatomical structures, we considered the Hausdorff distance ( d H ), that computes the worst possible Euclidean distance ( d ( p i ,  p j )) between two sets of points P f (| P f | =  N ) and P m (| P m | =  M ). We also considered the average of the minimum Euclidean distances ( d E ) to express global similarity between structures defined by landmarks.

Area-based metric

To evaluate the superposition between two anatomical structures defined as 2D landmarks (points), we first computed their concave hulls and then considered their Dice similarity coefficient (polygon DSC).

Image similarity metric

For completeness, we also considered the structural similarity index measure (SSIM) as a pixel-intensity-based metric.

Comparison of coarse registration approaches

Figures  3 , 4 and 5 summarise the results with boxplots and Wilcoxon signed-rank tests for all the registration methods and metrics considered. Wherever possible, the results for different anatomical areas are presented separately. For the Euclidean and Hausdorff metrics, lower values indicate better registration, while for the SSIM and Dice metric higher results indicate better registration. Moreover, a qualitative example to illustrate misalignments between the reference points and the registered landmarks is provided in Fig.  6 .

Quantitative results for all the methods on each structure (Hausdorff distance, d H , lower values indicate better registration). The upper part of each boxplot figure indicates the results of pairwise statistical Wilcoxon tests: (ns: 5.00e-02  < p ≤ 1.00e+00, * 1.00e-02  < p ≤ 5.00e-02, ** 1.00e-03  < p ≤ 1.00e-02, *** 1.00e-04  < p ≤ 1.00e-03, **** p ≤ 1.00e-04).

Quantitative results for all the methods on each structure with multiple points (polygon DSC, higher values indicate better registration). The upper part of each boxplot figure indicates the results of pairwise statistical Wilcoxon tests: (ns: 5.00e-02  < p ≤ 1.00e+00 * 1.00e-02  < p ≤ 5.00e-02, ** 1.00e-03  < p ≤ 1.00e-02, *** 1.00e-04  < p ≤ 1.00e-03, **** p ≤ 1.00e-04).

Quantitative results for all the methods according to the SSIM metric (higher values indicate better registration). The upper part of each boxplot figure indicates the results of pairwise statistical Wilcoxon tests: (ns: 5.00e-02  < p ≤ 1.00e+00, * 1.00e-02  < p ≤ 5.00e-02, ** 1.00e-03  < p ≤ 1.00e-02, *** 1.00e-04  < p ≤ 1.00e-03, **** p ≤ 1.00e-04).

Qualitative example of the alignment between the reference image and a randomly selected subject (11). The background image corresponds to the warped image after registration, circles denote the reference points for each structure, crosses represent the registered points and lines are used to illustrate misalignment between the reference and the registration.

Regarding point metrics, the ellipse ( E ) and ellipse with affine methods ( E+A ) obtain overall better results than the other methods. In general, the differences observed were found to be statistically significant for both point metrics and most anatomical structures. Exceptions to this are the thalami and the cerebellum where the pixel based affine registration method initialised using the reference ellipse ( AFF+I ), obtains results that appear worse but are not significantly different. Comparing the E and E+A methods, small (and not statistically significant) differences can be observed. Using a refinement registration after ellipse-based method slightly worsens the metrics when applied to the skull but improves them in all other anatomical structures. This is an expected result as the main focus of the first method is to co-register the skulls (ellipses). The uninitialised affine transformation ( AFF ) obtains significantly worse results than these two methods and has a higher variance of values as illustrated by its wide boxplots. Both methods using exclusively pixel-wise affine registration (AFF, AFF+I) achieve results that are worse than the metrics of the original moving image in some cases. This illustrates the disconnect between pixel-based metrics guiding these methods and point-based metrics targeting distances between real anatomical structures (especially for noisy sequences).

Regarding the differences between the Euclidean and Hausdorff metrics (especially the median values shown by the central line in boxplots), slightly higher Hausdorff values indicate that some point pairs are further than the average euclidean distance mean value. This is especially noticeable on the skull for E and A+E and the thalami for AFF .

The other two metrics show the same general tendencies, even though they focus on different aspects of the registration. The low SSIM values observed for all methods (even though E and A+E outperform all methods) illustrats how challenging this registration scenario is. The variations between individuals and acquisitions and low SNR make the intensity values particularly unreliable. On the other hand, the high anatomical DSC results observed for the ellipse-based methods (includding AFF+I) validate our geometric approach that relies on a rough initial skull estimate to determine the general shape and orientation of the brain. Similarly high results obtained for the cerebellum indicate that a correct skull placement is a crucial important step towards the registration of all brain structures.

Finally, Fig.  7 shows the final heatmap generated from the registered concave hulls (E+A) of the cerebellum for all output images. Even though faint outlines of some incorrect registration results outside of the cerebellum region (delimited by orange points) can be observed, the higher probability regions in the heatmap clearly correspond to the cerebellum region of the reference image delimited by the manually annotated orange landmarks.

Probabilistic map for the cerebellum structure based on the averaged concave hull of the landmarks for each image. Landmarks for the reference image (background) are also provided.

Usage Notes

The code provided to analyse the images and perform a rough initial alignment to a common space has been developed using python. A set of Jupyter notebooks detailing the use of each step is also provided in the repository with visualisation examples of each step of the processing pipeline. Furthermore, we provide the trained weights for the skull segmentation network as part of the data records (file unet.pt 19 ).

Regarding the use of python packages, the code heavily relies on pytorch (version 1.12.0 with CUDA 10.2) to do the heavy lifting. However, numpy (version 1.21.6), scipy (version 1.7.3), scikit-learn (version 1.0.2), scikit-image (version 0.18.1), pandas (version 1.3.4), persim (version 0.3.1) and alphashape (version 1.3.1) are also used for some of the processing steps or to analyse different metrics related to the registration. Furthermore, opencv-python (version 4.5.2.52) and gimpformats (version 2021.1.4) where used to open the images with python. In particular, manual landmark annotation where done using the Gimp software and were stored (together with the ultrasound image) using the xcf format. Finally, matplotlib (version 3.7.1), seaborn (version 0.11.0) and statannot (version 0.2.3) were used for results visualisation inside the Jupyter notebooks.

Code availability

All the code used in the study to generate the final atlas and to co-register the images is publicly available at https://github.com/marianocabezas/fetal-brain .

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These authors contributed equally: Mariano Cabezas, Yago Diez, Clara Martinez-Diago, Anna Maroto.

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Brain and Mind Centre, University of Sydney, Sydney, Australia

Mariano Cabezas

Faculty Of Science, Yamagata University, Yamagata, Japan

Hospital Universitari de Girona Doctor Josep Trueta, Girona, Spain

Clara Martinez-Diago & Anna Maroto

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All authors conceived and designed the experiments, C.M. and A.M. collected the images and performed the landmark annotations, M.C. and Y.D. conducted the experiments and analysed the results. All authors reviewed the manuscript.

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Cabezas, M., Diez, Y., Martinez-Diago, C. et al. A benchmark for 2D foetal brain ultrasound analysis. Sci Data 11 , 923 (2024). https://doi.org/10.1038/s41597-024-03774-3

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Received : 27 December 2023

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DOI : https://doi.org/10.1038/s41597-024-03774-3

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