Myth Monday: Big Babies

02/22/2021

As you near the end of pregnancy, you may start to wonder, "How on earth will I get this baby out?! Can I grow a baby that's TOO big?" In most cases, the answer is no. Unless there are other risk factors, such as uncontrolled gestational diabetes, it is rare that your body will grow a baby too big for your pelvis. Despite this, approximately 1 in 3 birthing people will be told that their baby is too big, but only 1 in 10 are actually born big. Let's talk about the "Big Baby Lie."

How do I know if my baby is too big?

There's no accurate way to know how big your baby is until it is born. If your provider is recommending ultrasounds late in pregnancy to check on your baby's size, this is a huge red flag. In cases where your provider thinks your baby is too big, you might hear that it is measuring a few weeks ahead of schedule, or they may want to change your due date. But ultrasounds performed after 22 weeks should not be used to predict baby's size because the growth of each baby is not predictable (unlike the first trimester) and growth can differ by about 2-3 weeks, even in average sized babies. Ultrasounds results can be up to 15% above or below the actual weight of your baby. Due to this, ultrasounds are only right about half the time when they predict a big baby. 

Why is my provider concerned about a big baby?

In most cases, doctors are concerned about two things: Cephalopelvic Disproportion (CPD) and Shoulder Dystocia.

CPD occurs when a baby's head is too big to fit through the pelvis. For first-time, low-risk births, CPD is the most common diagnoses for a first cesarean, but it is actually very rare in industrialized nations (only 1-2% of all diagnosed cases are genuine). More often, CPD is misdiagnosed when there is a "failure to progress" in labor; in these cases, the more likely problem is that the baby is not ideally positioned, or the pelvis is misaligned. Instead of recommending position changes, encouragement and emotional safety, or manual repositioning of the baby, most hospital providers will jump straight to a cesarean. 

Shoulder dystocia occurs when the baby's head has been born, but the shoulders are stuck inside the pelvis. This is a very serious condition because the chest compression while being stuck in the birth canal stops blood flow from leaving the head, leading to intercranial bleeding, brain damage, and death unless it is resolved quickly. 7 to 15% of "big babies" experience some level of difficulty in the birth of their shoulders, but most of these cases are resolved by the care provider with no significant harm. According to Evidence Based Birth, "Permanent nerve injuries due to stuck shoulders happen in 1 out of every 555 babies (0.1%) who weigh between 8 lbs. 13 oz. and 9 lbs. 15 oz., and in 1 out of every 175 babies (0.5%) who weigh 9 lbs. 15 oz. and greater. When interviewing your provider, ask questions about the training they have received in resolving shoulder dystocia and get a good idea of their routine practice in managing this potential complication. Shoulder dystocia can occur in average sized babies, too; so whether you have a suspected big baby or not, this is a good discussion to have!

My doctor says my baby is too big. Now what?

Studies show that just the "suspicion" of a big baby increases your chances of a cesarean. If your provider thinks that your baby is too big, they may be less likely to work hard to preserve a vaginal birth, and more quickly diagnose a failure to progress if your labor stalls. Unless there is a medical indication for ultrasounds late in pregnancy, a less is more approach can help you avoid this suspicion. 

In order to prevent CPD and/or shoulder dystocia, your provider may recommend an early induction or cesarean. However, early inductions (at 37-38 weeks) haven't shown to improve outcomes for big babies in relation to nerve damage or NICU admissions associated with shoulder dystocia.  Because we know that ultrasounds are not a reliable method to predict a baby's size, ACOG (American Congress of Obstetricians and Gynecologists) says that cesareans "may" be considered only if the estimated weight of the baby is 11 lbs. or greater. 

It's important to take into account the risks associated with inductions and cesareans and weigh them against your individual risk of CPD and shoulder dystocia.


Did your provider predict that your baby would be "too big?" Tell me about your experience in the comments!


Information in this blog should not be taken as medical advice. Any questions regarding your care during pregnancy, birth, or postpartum should be discussed with your provider.