How Accurate Are Ultrasounds?

By Sandra Everets

July 7, 2020

I'm a Mama of two beautiful children, Evelyn & Noah. I've worked in Obstetrics for the past 14 years. I've have the amazing pleasure of working with women and their new families through their pregnancy, labour and delivery & post partum. I look forward to getting to know your family.

Oh Baby! How big is your baby going to be? How accurate are ultrasounds?

As we approach our due date this question lingers in our mind as we ponder how our labour and delivery might go and if our baby will fit in our birth canal.  I remember trying to pick out baby clothes in the store and wondering how big my babe was going to be – did I need to buy newborn clothes or if she was over 7lbs could she just fit into 0-3 month clothes? You also might have been told your baby is larger than average and you are hoping that ultrasound is wrong. 

Let’s talk about how accurate ultrasounds are and what goes into calculating your baby’s Estimated  Fetal Weight (EFW).  Your baby will be measured to calculate their EFW at 18-22 weeks during your anatomical ultrasound and this helps your healthcare provider determine if your baby is growing along the growth curve appropriately.  If you have a health pregnancy and a normal anatomical ultrasound with no concerns you may not have another ultrasound during your pregnancy.  Your healthcare provider may decide to send you for an ultrasound in the last couple weeks of your pregnancy or at/after your due date and an EFW will be calculated. High risk pregnancies, women with Gestational Diabetes (GDM), and other risk factors will have more frequent ultrasounds and evaluations of their baby’s growth.  Your baby’s EFW is calculated by using a fancy calculation of 4 different measurements. First, we measure your baby’s head circumference (HC) and their biparietal diameter (BPD; the measurement of the skull from one parietal bone to the other). Second, we measure your baby’s abdominal circumference (AC) and finally the femur length/thigh bone length (FL). These four measurements are put together into standard tables/calculation programs to determine your baby’s EFW. 

Currently, even the best Estimated Fetal Weight calculation can give an error of plus or minus 15%.  As your baby grows and you get closer to your due date getting this measurements can be more difficult – the head is sitting low in your pelvis, baby’s abdomen may be more squished due to position/room and baby is practicing her breathing or having hiccups. There are many other factors that might increase the rate of error such as body habitus, amniotic fluid, number of babies, etc.  If you are told your baby is EFW is lower or higher than you expected please speak directly to your healthcare provider and have a dialogue about the accuracy of ultrasounds, your specific pregnancy and what that means for the remainder of your pregnancy. And remember it’s an Estimated Fetal Weight.

If you have any questions, reach out to Mama Coach in your area across North America and we’ll be happy to support you along in your pregnancy and postpartum!

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