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Measuring Baby’s Weight at your Third Trimester Ultrasound

Measuring Baby’s Weight at your Third Trimester Ultrasound

Written by Katie Hoffman, Certified Sonographer at Center for True Harmony Wellness & Medicine

Third trimester ultrasounds are used for evaluating things like amniotic fluid, the placenta, and some of Baby's anatomy, but one of the key pieces of information that these ultrasounds can provide is an estimation of Baby's size. As a sonographer, I am frequently asked if I can tell how much someone's baby weighs, and the answer is actually a little more complicated than one might expect. Yes, the measurements that are obtained during the ultrasound are used to calculate an estimated weight, but is that truly the correct size of the baby? 


You may have heard that these growth ultrasounds are not incredibly accurate, and the truth is that even when the measurements are extremely precise, the estimated weight can be different from the baby's true size by +/- 1 lb. A study I read recently explained why there can be such a large difference.


In this experiment, researchers placed new born babies in water in order to evaluate their mass compared to their birth weight. They measured the amount of water that was displaced, and they found that babies who had the exact same weight could in fact be different sizes. Or in other words, babies who were the same size (who displaced the same amount of water) could have different weights (up to 500 g or 1.1 lbs). It was determined that various factors, such as muscle mass and bone density, largely affect a baby's weight. When it comes to third trimester ultrasounds, the fetal biometry measurements that we obtain are actually better at evaluating a baby's mass. The estimated weight that is calculated is just an estimate based upon the size (or mass) of the baby. But since those extra factors (bone density, etc.) cannot be evaluated with ultrasound, the TRUE weight of the baby cannot be measured with 100% accuracy.

Other factors can widen that margin of error even more, such as the time between the ultrasound (at our office we perform this study around 36 weeks gestation) and the birth of the baby, growth rates, the formulas used (the ones used by the ultrasound machines are more accurate for babies around 6-7 lbs, and become less accurate when the baby begins to approach larger sizes, like 9-ish lbs). Even though ultrasound might not give you the EXACT weight of your baby, it can still be close! And it's our goal as sonographers to measure as accurately as possible so that the estimated weight can be as close as possible to Baby's true size.




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