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.

 

 

 

Author
True Harmony Provider The True Harmony providers collaborate to bring easy to understand education to our community

You Might Also Enjoy...

Importance of Iron

Understanding the iron in your body when you're pregnant is important. This post goes into detail on signs to look out for and helpful tips to how your diet can contribute to a healthy iron count. Read on to learn more!

When to make an Appointment?

True Harmony wants to dive in head first for your breast health. In this post, we will be reviewing when to look into making an appointment with breast exams. Your breast health is very important and we want to make sure you're taking the right steps!

What to Expect at Your First Prenatal Appointment

Your pregnancy test came back positive! You are probably wondering what happens next, and when you should get started with your prenatal care. At True Harmony, our OB and midwifery team are excited to meet you and get you started on your pregnancy journey!

What to Know Before Getting Pregnant

If you are starting to think about pregnancy, whether that be in a few months, a few years, or just “sometime in the future,” then there are already steps you can take to prepare in order to set yourself up for a healthy pregnancy!

What is a dermoid cyst?

Ovarian cysts are very common in women, but did you know there are different types? You might find one of these types particularly interesting!