Large baby poses risk to mom, self

Dear Dr. Roach: My 31-year-old daughter is pregnant with her first baby. She is in her fifth month, and the doctor has told her that her baby is at the 75th percentile in growth. Can you please advise if this is a concern?

Answer: The 50th percentile is, by definition, an average-sized baby (for the appropriate population). The 75th percentile makes her baby a bit bigger than average but, by no means, abnormally large.

We start to worry when the baby is above the 90th percentile, although the 95th and 97.75th percentile are commonly used by researchers to define “macrosomia” — a baby who is larger than expected.

When a fetus is that large, there are risks to the mother, including protracted labor, the need for a caesarean birth, and damage to the genital tract and uterus.

Risks of a very large baby also include damage to the shoulder during delivery.

When a baby is that big, there is a concern that the mother may have high levels of insulin, which acts as a growth hormone for the baby.

Moms who gain excess weight during pregnancy are at a higher risk for larger babies.

There are other rare causes. All expectant mothers are screened for the form of diabetes that is associated with pregnancy.

A baby born to a mom with diabetes is at risk for low blood sugar when they’re first born.

However, your grandchild is not in a range where any of these bad outcomes are likely.

If your daughter or the baby’s father is a large person, there is simply a higher chance that they’ll have a large baby.

On the other side, a developing fetus that is less than the 10th percentile (but especially less than the 3rd percentile) for weight is a bigger concern because there are many worrisome causes for the baby not to grow. Her baby seems to be in a very healthy range.

Finally, the ultrasound pictures aren’t perfect and can under- or overestimate fetal size, so I really wouldn’t get too concerned.