Why have humans not evolved away from dangerous childbirth?

A mathematical model suggests maybe it’s not possible.

Humans have a relatively hard time giving birth. Compared with other species, human newborns are large for their birth canal. This means that humans face the surprisingly high risk of our babies getting stuck during labor—which can be fatal for the mother, the newborn, or both. Some estimates suggest it occurs in as much as six percent of births worldwide.

The fact that this is an ongoing problem for our species has been a puzzle for evolutionary scientists. Why have evolutionary pressures not pushed the species away from such a common and fatal situation? According to an international research team including biologists, a philosopher, and a pediatrician, the answer lies in a complicated push and pull of several evolutionary factors between the baby and mother. But, the researchers find that there's also a chance for us humans to tip our own evolutionary scales—with C-sections. Using a mathematical model, published in the Proceedings of the National Academy of Sciences, the researchers find that, in time, modern medicine could nudge human evolution off course, delivering bigger babies.

Falling off a cliff

To begin with, the sticking point is that there are competing evolutionary pressures for both the mother and the newborn, according to the study authors led by biologist Philipp Mitteroecker at the University of Vienna, Austria. Humans have big brains, which is a great thing, and you’re more likely to survive in life if you have a big brain (and a high birth weight). However, you’re more likely to survive birth if you’re small enough to fit through the birth canal. This means that the evolutionary pressures operating over an entire population push in the direction of larger babies—until suddenly, large becomes too large and the survival rate plummets.

At the same time, there appears to be some kind of evolutionary pressure keeping human pelvises small. It’s clear that such a pressure must exist, otherwise mothers with larger pelvises would simply be more likely to survive childbirth and larger pelvises would start to become more common among humans.

One explanation for small pelvis size has been bipedalism. Humans were already walking upright before our brains got this big, and so larger-brained babies were birthed through pelvises that were already adapted to bipedalism. Some researchers have suggested that a wider pelvis wouldn’t be suitable for bipedalism, but the evidence for this is limited. Another explanation is that a narrow pelvis is more likely to prevent organ prolapse—i.e. to keep guts from slipping out—and to support a pregnancy.

So, at the same time that selection pressures are pushing for larger babies, they’re pushing for narrower pelvises. This looks much the same as the pattern for newborn size: selection pressures acting over the whole population push toward smaller pelvises, until suddenly small becomes too small and childbirth becomes dangerous. The researchers describe this pattern as a “cliff edge,” because the chance of survival increases steadily until suddenly it plummets.

If you take 100 births and compare the size of the newborn and the birth canal, there will be lots of different degrees of difficulty and danger. In a small minority of cases, the newborn will be on the small side and the pelvis will be large, and the birth will be comparatively easy. In the majority of cases, there’ll be a larger newborn and smaller pelvis, and the birth will be more difficult. And at the other end of the scale, another small minority will have a newborn that’s too large, a pelvis that’s too small, and a dangerous obstructed birth.

In their mathematic model, Mitteroecker and his colleagues found that the cliff edge pattern explains the puzzle. The small minority of dangerous births persist because it’s not possible for a species to wholly adapt to these competing pressures—evolution is pushing in too many different directions at the same time.

C-sections offer a way out

Luckily for humans, C-sections offer a way to circumvent this evolutionary problem. Since the middle of last century, the procedure has been growing in safety and availability. The researchers also explored whether the growing availability of C-sections might reduce the pressure to have smaller babies, resulting in larger babies and an ever-increasing need for C-sections over time. Their model suggested that maybe they have done so already.

This conclusion has generated headlines reporting that C-sections are changing human evolution, but it’s more tenuous than that. Because the evidence comes from a mathematical model, its conclusions are more like a prediction than a declaration; the findings suggest that if we look in the real-world data, there’s a chance that this is what we might find. However, a model also has to make a lot of assumptions to get to a conclusion like this.

The researchers started with a conservative estimate: they assumed that just three percent of births are obstructed (and therefore need a C-section). If things like head size are indeed passed on genetically from one generation to the next (rather than being caused purely by factors like nutrition), their model suggested that over two generations of C-sections, we could expect to see a 20 percent increase in the number of births that would be obstructed. That takes us from three percent of births up to 3.6 percent.

It’s difficult to check this in real-world data, because C-section rates are way higher than 3 percent already, for a variety of reasons. And of course, there could be things other than obstructed labor keeping babies small, like the capacity of a mother's body to grow and sustain the baby. However, it does mean that maybe the ruckus about the best rate of C-sections has a new factor to consider: perhaps humans will start to need them more and more over time. It’s a neat example of how evolutionary pressures are still acting on our species today.