How C-Sections Could Alter the Course of Human Evolution

In the United States today, about 1 in 3 babies are delivered via Caesarean section (C-section)—a 50 percent increase since the mid-'90s. What accounts for the dramatic rise? It stems partly from the fact that many women are drawn to the convenience the procedure affords: They can control not just when birth will occur, but who the surgeon will be. But it's also a function of doctors practicing defensive medicine. When fetal monitoring suggests anything at all out of the ordinary, obstetricians often push for C-sections in order to avoid getting sued for failing to provide aggressive care.

Regardless of the reason, scientists are starting to realize that our increasing reliance on C-sections might be having some unintended side effects—ones that may be changing both the health of our children and, incredibly, how the human body itself evolves.

First, a growing body of research has found that children delivered by C-section don't fare quite as well as children delivered vaginally in terms of their physical and psychological health. Among other things, studies have found that delivery by C-section is linked to contracting more infectious diseases, higher rates of childhood obesity, as well as greater odds of developing autism.

That's not to say that a child delivered by C-section will necessarily be in worse health. Most, in fact, are perfectly healthy—it's just that their relative risk of developing certain health conditions seems to be modestly higher. For instance, with respect to autism, studies suggest that children born by C-section are 1.26 times more likely to develop it than children delivered vaginally.

So what exactly is going on here? Why do kids born via C-section tend to have more health issues? Scientists believe that it may be due to the fact that C-sections expose babies to different kinds of bacteria during delivery compared to vaginal births.

In a 2010 study published in the Proceedings of the National Academy of Sciences, researchers took bacterial samples from mothers' skin, mouth, and vagina shortly before giving birth, as well as samples of their infants' skin and mouth shortly after birth. About half of these children were delivered vaginally, while the other half were delivered by C-section.

It turned out that the children delivered vaginally had bacterial colonies that resembled those found in their mothers' vaginas. Many of these colonies consisted of "good" bacteria—that is, bacteria that helps us in some way, such as by making it easier to digest milk, or for our blood to clot.

By contrast, children delivered by C-section had fewer beneficial bacteria. Instead, their bacterial colonies looked a lot like the ones on their mothers' skin, which included a lot of staph—a type of bacteria that can potentially be dangerous.

Scientists believe that the bacterial composition of the vagina changes during pregnancy so as to expose babies to bacteria during delivery that will assist in healthy development. When delivery occurs by C-section, though, babies bypass those bacteria entirely and are exposed to different kinds that might not be so helpful. This might be what explains their higher risk of developing health issues. For instance, maybe the heightened risk of obesity is a function of these children not being exposed to certain bacteria that play a role in regulating appetite.

The rise in Caesarean deliveries may also have implications that go well beyond bacterial exposure and health. In fact, in a recent paper published in the Proceedings of the National Academy of Sciences, scientists argue that C-sections could potentially be changing the way our bodies develop, too.

Because the birth canal is barely big enough to permit childbirth as it is, a pregnant woman who has narrow hips or who is carrying a fetus with a large head could potentially be headed for a life-threatening situation known as fetopelvic disproportion. In our evolutionary history, disproportioned hips and heads were often deadly for both mother and child. As a result, wider hips and smaller heads became evolutionarily favored, meaning the genes that code for these body features were more likely to be passed along.

Today, however, fetopelvic disproportion is no longer a major concern in developed countries thanks to C-sections. This procedure allows doctors to safely deliver babies who might not otherwise make it. But the fact that we're doing this is effectively easing selection pressures for narrow hips and big heads. Consequently, we might very well be changing the way the human body itself is evolving, such that women's hips might start getting narrower, while babies' heads could get larger.

To be clear, this idea is speculative—we don't necessarily have firm evidence that it's occurring, at least not yet. However, the scientists who advanced this theory developed a mathematical model to estimate the extent to which fetopelvic disproportion has likely increased since the 1950s, when doctors started performing C-sections regularly and safely. By their calculations, the rate has risen by 10 to 20 percent.

With all of that said, the take-home message here isn't that C-sections are inherently a bad idea or that women should stop getting them. In cases of fetopelvic disproportion or fetal health emergencies, C-sections can be a lifesaving procedure. The point is simply that by changing how babies are delivered, we may also be changing the human body in ways that no one ever anticipated or intended.

Justin Lehmiller, PhD is the Director of the Social Psychology Program at Ball State University, a Faculty Affiliate of The Kinsey Institute, and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.