Should You Worry About the Size of Your Bump—and Your Baby?

Our maternity nurse weighs in on what it means if your baby is measuring small during ultrasounds.

Pregnant person holding an ultrasound image over their belly

Natalia Deriabina / Shutterstock

As a maternity nurse, I get questions all the time based on inaccurate or insensitive stuff friends and family say that freak pregnant people out. C'mon guys, leave these poor expecting parents alone. If they're getting good prenatal care, let their doctor be the one to voice concerns.

One mom-to-be was worried that her bump—and her baby were measuring small at 33 weeks. At 3 pounds at that point, the baby is certainly measuring smaller than average. We'd expect her little one to be closer to 4 pounds at this point.

However, a single ultrasound doesn't always indicate accurate size and growth patterns. Observing growth over time will be important.

Will the baby start catching up on the growth chart or will they turn out to have intrauterine growth retardation (IUGR)? IUGR indicates a baby who's not growing well inside the womb. This condition is usually caused by a lack of nutrition or oxygen getting through the placenta to the baby. Sometimes, it's caused by smoking, poor nutrition, drug abuse, diabetes, or other medical diagnoses. Essentially, something is adversely affecting the placenta.

Sometimes, IUGR is the effect of preeclampsia, high blood pressure, or other circulatory conditions preventing adequate blood flow to and from the placenta. Sometimes, IUGR is caused by genetic anomalies. A lot of the time, we don't know why IUGR happens.

I expect what will happen next is that the doctor will repeat ultrasounds frequently and start monitoring the baby's well-being with nonstress tests and a fetal heart monitor, either in the doctor's office or at the labor and delivery unit of the hospital. They'll look for specific heart patterns that indicate good fetal circulation and oxygenation and monitor the baby's growth over time.

They may have other tests done as well, like blood work to rule out serious issues like high blood pressure or other complications. If all of these tests turn out reassuring, the doctor will probably just wait and see what happens.

If tests start indicating that baby's in trouble, they'll start talking about contingency plans like bed rest to reduce circulatory stress and check-ups with a perinatologist (who specializes in fetal medicine and premature babies).

In the uncertain but less common scenario that the baby is really doing well, they'll talk about the possibility of premature delivery. This doesn't happen very often. Most of the time, the baby is growing well enough inside the expecting parent that they leave them there. When a baby is born, their pediatrician will keep a close eye on them to make sure all systems work well. Most of the time, they're fine. Smaller than average but fine.

That's a whole lot to worry about but most of the time smaller than usual babies are just that—little ones. Maybe the baby's little because the expecting parent is little. That's why they invented petite sizes. Maybe the parent isn't as far along as previously thought and the baby's size is actually normal for a 31-weeker. Maybe they're just about to hit a growth spurt and will be right where they're supposed to be in another week or so.

If I can offer one useful piece of advice it's this: When people offer you unwanted comments or advice on your pregnancy, stick your fingers in your ears and sing, "LALALALALA! I can't hear you!"

If that's not your style, try saying, "Thanks but I've got a great doctor who's taking good care of me. I'm fine."

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