Theoretical biologist Philipp Mitteröcker is intrigued by the puzzle of dangerous human childbirth. Unlike other species, human babies are often too big for the birth canal, leading to dangerous—and possibly fatal—obstructed labor. Last year, Mitteröcker and his colleagues published a mathematical model that showed how the mixture of evolutionary pressures acting on humans would inevitably lead to an ongoing risk of obstructed labor in our species.

The model also suggested that C-sections are changing the rules of the game by increasing the likelihood that large babies and their mothers survive childbirth and pass on genes that promote this head/pelvis mismatch. The model predicted that we'd see an increasing risk of obstructed labor (and need for C-sections) over generations—but there was no real-world evidence of that happening.

Now, in a new paper, Mitteröcker and colleagues have published empirical evidence that this is indeed the case: women who were born by C-section seem to have a higher risk of needing a C-section themselves. And the real-world increase in risk is similar to what their model predicts.

How many C-sections, and why?

That conclusion sounds cut and dried, but the evidence is far muddier than it might seem. For a start, the rate of C-sections in industrialized countries is far higher than the rate of obstructed births. There are other medical reasons to have a C-section and plenty of non-medical reasons, too. Most medical data just records the rate of C-sections and not always the reasons.

Even if the data does record how many of the C-sections were due to obstructed births, not all obstructed births are caused by the mismatch in baby size and pelvis size (called fetopelvic disproportion, or FPD). Even if medical data sometimes tells us which C-sections were needed because of obstructed labor, it won't tell us how many cases of obstructed labor were caused by FPD.

All of this means that it’s difficult to figure out how many C-sections were a result of a head/pelvis mismatch and whether this rate might be increasing over time. Which is the only thing relevant to the specific evolutionary pressures in Mitteröcker's model.

Finally, there’s the question of how much of this risk can be passed down genetically. Environmental factors like nutrition and cultural factors (like norms surrounding birth) are likely to play an important role—and there’s the additional complication that any baby is also carrying its father’s genes. The picture is pretty tangled.

Intergenerational risk

But even this messy data could yield clues if you look in the right place. Mitteröcker and his colleagues went in search of cross-generational data that could show whether rates of this pregnancy complication might be increasing.

A Swedish dataset of more than 2.5 million people suggests that mothers were 1.66 times more likely to have an obstructed labor if they themselves had been born following obstructed labor. In other words, if the general risk of this problem hovers around two percent, these women had a risk of around 3.32 percent. This is a lower increase than the model, which predicts an increase from two percent to around five percent. The difference can possibly be explained by the messy real-world data, compared to the simplified assumptions of the model.

A study done in Norway also found an inherited increase in risk between mothers and daughters, but no link through the paternal side. That is, fathers who were born by C-section didn’t seem to increase the risk of this complication for their partners. That’s a bit confusing because, presumably, fathers would pass on genes that contribute to the size of the baby.

Mitteröcker agrees that it’s a surprising result: “We would expect the father’s genome to play a role, but I don’t know if the data we have is fine enough to see that,” he told Ars. “The size of the baby is also environmentally driven via nutrition and depends on the mother’s physiology.”

Without knowing more about how, exactly, genetics contributes to this issue, predicting how the father’s genome would be expected to play a role is challenging.

Does this mean more C-sections are a bad thing?

There’s currently a ruckus about the high rate of C-sections in the industrialized world, with many people pointing to the potential harms of unnecessary surgical births. People might try to use this paper to claim that a high rate of C-sections is increasing the risk of childbirth, but that would be a misunderstanding of the evidence.

If a mother chooses to have a C-section for non-medical reasons, there’s no reason to think she had an especially high risk of complications in the first place. So there’s no reason to think that she’s passing on a higher risk of dangerous childbirth. Similarly, C-sections done for medical reasons other than fetopelvic disproportion shouldn’t increase the risk for subsequent generations.

The increase in risk comes only when a C-section saved the life of a mother, baby, or both when they would otherwise have died in childbirth. That mother and baby have the chance to live, which includes potentially passing on the genes that mean a greater need for C-sections in the future. Because we have the medical technology to handle this, this isn’t a detrimental change—and it’s a relatively small increase. There’s no reason to think that all humans in the future will be born by C-section.

Despite the uncertainties in the recent study, we can draw some lessons. For one thing, it’s probably worth getting more detailed statistics on obstructed labor so we can understand the issues better. It’s also reasonable for doctors to consider family risk factors in childbirth.

Mitteröcker thinks the evidence could help us to think about how to distribute research funding. Because excellent medical care is improving the chances of smaller babies, the loss of selection against large babies might also be diminishing, counteracting the effect of the C-sections. “We should support research in neonatology to ensure that smaller babies have good survival rates,” says Mitteröcker. Likewise, he says, larger pelvises might be lower-risk for childbirth, but they come along with a higher risk of pelvic floor disorders, like organ prolapse. “There should also be investment into research into pelvic floor disorders.”

This research is still in its infancy. There are plenty more questions to be answered—about the genetics of fetopelvic disproportion risk, the role that neonatal care might play in helping smaller babies to survive, and whether other data sets show the same increase in risk. But the real-world evidence supports the original mathematical model, and so it helps us to understand why we’re in this sticky evolutionary situation at all.

PNAS, 2017. DOI: 10.1073/pnas.1712203114 (About DOIs).