Up here in the mountains “big” babies aren’t as common, but many mothers – locally and nationally – are either induced or recommended to have a cesarean section for a suspected “big” baby. Similarly, some women are induced due to a suspected “small” baby. As doulas, we are often supporting families who are making a decision about when and how to birth their baby that they’ve been told is either too big or too small. It got us thinking about trusting our bodies and wondering how accurate prenatal measuring techniques are.
Over millennia, human female bodies have been evolving to allow human babies to gestate long enough that they are likely to survive once born. When our ancestors started walking upright, our bodies shifted, re-configuring the pelvis to allow a relatively large baby to be born. Throughout pregnancy, women’s bodies release a hormone called relaxin that helps loosen connective tissues, providing more stretch and movement in our skeletal systems. It does not behoove the human race to grow a baby that cannot get out. Does that mean nature never messes up? No, but it does mean that the vast majority of women will grow babies that they are physically able to birth. This is part of what we mean when we say “trust your body.”
The reason that care providers often give when they recommend an early induction or a cesarean for a suspected big baby is that they want to avoid the baby getting stuck, generally by a shoulder dystocia. But the tools currently available for measuring babies while they are in utero are drastically inaccurate. Typically, these measurements come from ultrasound, and even the most accurate formula (Warsof2) for estimating weight (which is not necessarily employed by all care providers) still has a +/- 1 pound margin of error (generally, weight estimations by ultrasound are only about 50% correct). Shoulder dystocia, when the baby’s head is birthed but the shoulders get stuck, is a legitimate concern for big babies. However, most shoulder dystocias are handled by the care provider with no harm done to the baby. A risk of shoulder dystocia is nerve damage to the arms, called brachial plexus palsy. And although the rate of brachial plexus palsy goes up with an increase in the weight of the baby, the percentage of babies who have permanent damage is consistent throughout all weight ranges.
So, women’s bodies are designed to birth babies and our current methods for measuring babies in utero are inaccurate. One might start to believe that this is an example of the over-medicalization of birth. If you or your friend/sister/cousin is in the situation of having to decide about an early induction or cesarean due to a suspected big baby, look at the evidence and trust your body.
For a thorough review of the evidence on suspected big babies, read the Evidence Based Birth blog post. If you don’t have time to read the whole thing or get overwhelmed by the numbers, scroll down to the end of the article where there is a concise “bottom line.”