The 20 Week Anatomy Scan: How to Prepare and What You Can Expect

I’m currently pregnant with our first child and recently experienced the anatomy scan– as a patient this time! It was so incredibly special to see our baby as a full on baby vs the gummy bear we had seen months earlier. The experience really gave me a greater understanding of the weight this scan can hold for expecting parents and a couple of things I would appreciate knowing going into the scan if I weren’t a CNM. So, I’m breaking down the general need to knows for the anatomy scan in hopes that you can go into this scan confidently knowing just what the purpose of it is, the possibilities of findings, and ways you can communicate with your provider if you find yourself having questions after your own experience.  

Purpose 

The anatomy scan is the scan performed in pregnancy around 18-20 weeks. The scan is performed around this time because your baby’s organs, bones and body systems have developed to an extent where they can be visualized well with ultrasound. This assessment also helps to determine details that may influence the remainder of you and your baby’s care. 

Depending on the findings, the individual details of your pregnancy, and your providers’ recommendation, this may be the last ultrasound of your pregnancy so ensure to bring your support person of choice and soak in the images of your little one! 

How to best prepare for your scan

There’s a couple of things you can do the day of your scan that might help your ultrasonographer get the best images possible of your baby. One of those things is to have a full bladder at the time of the scan –not to the point where you’re uncomfortable but definitely on the fuller side. Thanks to the anatomy of our pelvic organs, having a full bladder will help to lift your uterus farther out of the pelvis for a better visualization during the scan. Eating a meal or nutrient dense snack about an hour beforehand can also help your little one’s activity level and allow for different angles and structures to be captured more easily. 

Despite your ultrasonographers’ best efforts and your preparation of eating a snack, it’s not always a guarantee that all the standard images of your baby will be obtained during your initial scan. This can be due to your baby’s position, your uterus’ position or your body habitus. If this occurs, it’s common to return to your office 1-2 weeks later in an attempt to reimage those missing views when your baby is perhaps in a different position and bigger in size. 

As a quick but important aside, it’s not in your ultrasonographer’s responsibilities to communicate with you whether or not everything is developing typically or if findings are normal or abnormal. Although they are more than capable of knowing this, it’s your provider’s responsibility to communicate these findings. In short, don’t take your ultrasonographers silence or ambiguity as concerning. 

Details of the scan

Your Baby’s Size

The scan not only captures images of your babies organs and bones but also takes measurements of certain structures which help to estimate yours baby’s size. This estimate is truly just that… a guess of how much your baby weighs based on the measurements of certain aspects of the head, abdominal circumference, humerus (arm) length, and femur (leg) length. This estimate and subsequent correlation to the bell curve does not indicate that your baby will chart in a similar fashion throughout the whole pregnancy or at the time of birth. It’s simply just a snapshot in time to identify any babies who are significantly lacking in growth and would need to be monitored more closely throughout the pregnancy. These babies generally are under the 10th% in their overall estimated weight. Additionally, this estimated size does not change your due date. Your due date is set in place by either your last menstrual period or an early first trimester ultrasound. More information on your due date and your baby’s size can be found here and here. 

Your Anatomy 

Your cervix will also be measured in this scan and is the lower aspect of your uterus that will eventually soften, shorten and dilate as you enter labor. However, at this point in your pregnancy, it’s important for your cervix to remain closed and long, serving as a protective barrier between your growing baby and the outside world. Cervical lengths measured to be greater than 2.5cm are considered a normal and reassuring finding that preterm birth is unlikely.  If you have fibroids (benign growths in various layers of the uterus), these will also be measured to ensure that they are stable in size and not impeding the growth of your baby. Lastly, your ovaries may also be visualized but aren't always considering your uterus’ new size. 

Baby’s Anatomy 

The major systems of your baby’s development are assessed starting with their head and neck which looks at many different elements in your baby’s brain. Aspects of their face including their nose, lips, and profile are also visualized (these tend to make for the sweetest pictures to take home!). Your baby’s thorax which includes the sternum, ribs, cartilage, and muscles surrounding these structures as well as the lungs are visualized. Your baby’s heartbeat and heart structure are also captured which aims to confirm there are 4 chambers within their heart. The abdomen includes the stomach, kidneys, bladder and genitals which can all be seen at this stage. If you chose to perform non invasive prenatal testing, you may know the sex of your baby before this scan. If you didn’t, this scan can reveal that! **Pro tip, if you are waiting to find out the sex of your baby until the time of delivery be sure to tell your ultrasonographer this prior to the start of your scan. The spine is additionally visualized all the way from the neck to tailbone of your little one. Lastly, their arms, legs, fingers and toes are assessed and counted. 

Baby’s lifeline aka the Placenta

Your baby’s placenta is the organ that develops around 10 weeks of pregnancy and provides nutrients and oxygen to your baby while removing waste from your baby’s blood. The organ attaches to your uterine wall and has a cord that inserts into your baby’s stomach. (this will later become the belly button after birth!). The anatomy scan can help to identify where in your uterus your placenta is located. It could be on the back side (posterior), the front side (anterior), the sides of your uterus (R of L lateral), the top of your uterus (fundal) or more rarely, touching or covering your cervix (partial previa or complete previa). Besides the last two variations (partial or complete previa) it does not matter where your placenta is located and will not impact details of your labor or birth. It can however impact the way in which you feel your baby, specifically for moms who have an anterior placenta. Because there is an additional layer between you and your baby with an anterior placenta, it may take a little longer to feel your baby’s kicks or they may feel less intense than movements from babies with other variations of placentation. The typical shape of your placenta is circular however there are variations to this too which can include an accessory lobe (small lobe off to the side) or a bilobed placenta (two equally sized round structures). 

The size of your placenta may vary but at this point in time, is not routinely measured at the anatomy scan. The insertion site of the cord however is observed. The cord can be inserted centrally (in the middle) as is most typical, closer to the edge (marginal) or into the membranes vs the tissue of the placenta (velamentous). If the insertion of your baby’s cord is a variation other than central, your provider will want to monitor your little one more frequently throughout the pregnancy. The umbilical cord itself typically has three vessels inside of it– 2 arteries and 1 vein. These vessels can be observed during your anatomy scan and a possible variation is that the cord has just 1 artery and 1 vein. In this case your provider may want to check in on your little one’s development more frequently throughout your pregnancy. 

Baby’s Position 

Although your baby’s position at this point in pregnancy means nothing at all, your ultrasonographer will observe and document it. The truth is, by the time you get into your exam room to talk with your provider, your baby’s position could have changed already! The variations are vertex (head down), breech (butt down) or transverse (laying sideways). Again, if you are desiring a vaginal delivery and your baby is not head down at this scan, don’t fret! This is very normal. Lastly, your little one is surviving in a pool of fluid known as amniotic fluid. This fluid will be measured to ensure there is an adequate amount and not too much or too little. 

Potential Variations 

As you might have gathered, there are many different aspects of your baby’s development, your anatomy, and surrounding details of the pregnancy that are assessed during this scan. While some variations are considered normal, there are others that may require additional testing, evaluation, or follow up. These variations can include referral to a specialized OBGYN also known as Maternal Fetal Specialists while other variations can just mean your baby may need to be measured more frequently throughout the pregnancy or assessed closely at the end of pregnancy. Regardless of the presence or absence of variations on your scan, it’s important you understand what all has been assessed and what the implications of the findings are, if necessary. Below are questions that may help you find clarification should you need it when reviewing your own scan with your provider. 

“Will any findings on today’s scan mean that more ultrasounds or tests need to be performed later in the pregnancy?” 

“Can you explain the variation you spoke about in more detail?” 

“Are there any restrictions or recommendations you would make for my lifestyle based on the findings of today’s scan?”

“Does this finding have any implications on the timing or the mode of delivery?” 

“Do you mind repeating that?” 

I hope that this blog post empowers you to be an active participant in your own care with a better understanding of what the purpose of this scan is and how you can best prepare. While I understand this scan can be anxiety producing for some, I hope you find peace in knowing what you can expect. If you find yourself preparing for your own scan, take a big deep breath, know (and believe) you are the perfect mom for this baby, and soak in the images of your little one. 

Always on your team,

Skyler

Sources:

O'Hara S, Zelesco M, Sun Z. Cervical length for predicting preterm birth and a comparison of ultrasonic measurement techniques [published correction appears in Australas J Ultrasound Med. 2013 Nov;16(4):210-211]. Australas J Ultrasound Med. 2013;16(3):124-134. doi:10.1002/j.2205-0140.2013.tb00100.x 

Reicher L, Fouks Y, Yogev Y. Cervical Assessment for Predicting Preterm Birth-Cervical Length and Beyond. J Clin Med. 2021;10(4):627. Published 2021 Feb 7. doi:10.3390/jcm10040627