Bigger babies? So what?

Taking a closer look at the claim that caesarean sections are driving evolution

In October 2016, a research paper prompted widespread news reports that caesarean sections are affecting human evolution by causing the size of newborns to increase. Larger babies which, in the past, would have died from obstructed labour are now able to survive. The alleles—gene variants—that cause this obstructive ‘fetopelvic disproportion’ (FPD) are no longer selected against and are, it is claimed, becoming more prevalent in the human population. However, the story was massively over-hyped as no solid evidence has been found.

The original study, published in PNAS by Mitteroecker et al., is a mathematical model that seeks to explain a longstanding evolutionary conundrum. Worldwide, a woman dies every two minutes from a complication of pregnancy or childbirth. FPD accounts for many of these deaths through the impossibility of natural birth; without medical intervention, both mother and baby will almost certainly die. Evolutionarily, this is puzzling. How is it that, despite near-certain death, babies continue to develop that are too large to be born? Shouldn’t natural selection have ‘fixed’ this?

Let’s imagine all foetuses are small enough that they can be born safely. Some are larger, some are smaller, but each is small enough that its birth won’t pose a problem. The larger ones, however, are ‘fitter’ than their diminutive peers, so over time the population will evolve to include more alleles encoding greater foetal size.

What if the average size of foetuses increases a little further? Now the largest foetuses run a risk of dying if they find themselves in the womb of a woman with a relatively small pelvis. However, due to variation in pelvis size, most will be fine, and these especially large newborns will enjoy even greater health than those who play it safe.

Mitteroecker et al. calculate that this improved health makes up for the risk of death, and that we should therefore expect a constant proportion of two to six percent of pregnancies affected by FPD. In short, the optimum baby size is slightly larger than can be safely delivered by all women, and this means that an evolutionary pressure continues to push us towards this optimum despite the fact that some unlucky mothers and babies will die.

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Are caesarean deliveries changing this balance? The availability of this intervention has greatly reduced the selection pressure against oversized foetuses in many parts of the world. Mitteroecker et al.’s model predicts that this will have caused a relative increase of between nine and 20 percent in the number of babies that are too large to fit through their mothers’ pelvises. This is the figure that has been seized on and wildly misreported by clickbait headlines.

However, this study relies on a theoretical model rather than real world data, and so does not report an actual increase in the observed rates of FPD. The news coverage of this research often reports an increase of incidence of FPD from 3.0 percent to 3.6 percent of pregnancies over the last half century. These figures do not appear anywhere in the original paper. Instead they seem to have been extrapolated from the relative nine to 20 percent increase predicted by the theoretical model. This really is a very serious bending of the facts.

The actual rate of FPD is difficult to measure, with estimates cited by Mitteroecker et al. ranging from two to eight percent of pregnancies. So we don’t know precisely what the real world rate of FPD is, let alone whether it has increased by less than one per cent in recent decades.

Much of the sensationalist reporting on this result has taken the angle that mothers ‘too posh to push’ are to blame, but this is misguided and irresponsible. It makes the patronising accusation that pregnant women opt in for major surgery on a whim, as well as entirely missing the point. The only individuals relevant to this evolutionary change are by defi nition those for whom a caesarean prevents death due to FPD—mothers with large babies and too small a pelvis. The rate of delivery by caesarean for pregnancies where the foetus is not oversized has no impact. This misrepresentation risks discouraging mothers and doctors from a caesarean delivery in cases when it would in fact be the best option.

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Perhaps the greatest mistake made when discussing the impact of modern medicine on human evolution is the implicit assumption that this is undesirable.

All changes to our environment will inevitably shape the direction of our future evolution. Natural selection involves, nay, requires the untimely deaths of individuals unlucky enough to carry faulty genes—in this case the mothers and babies aff ected by fetopelvic disproportion. Thanks to caesarean deliveries, women and children who would previously have died in childbirth are leading long and healthy lives. If that’s not progress, I don’t know what is.

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