If a newborn baby is too large or a woman’s pelvis too narrow, it can be dangerous or deadly.
In theory, these are the sorts of characteristics that evolutionary pressures might make less common over time.
And yet, as researchers point out in a study published December 5 in the journal Proceedings of the National Academy of Sciences, obstructed labour caused by these traits still occurs pretty frequently (3-6% of births worldwide, according to an analysis of World Health Organisation data).
The researchers conclude that there are certain evolutionary factors working against each other to keep these rates high. But more controversially, they conclude that more Cesarean section (C-section) births could be changing the balance of these factors and increasing the rates of obstructed labour. Still, there’s reason to be sceptical about how much we know here.
In the paper, the researchers explain that competing pressures may cause these high rates of obstructed labour in the first place. On the one hand, higher birth weights and brain sizes are advantageous to newborns, right up until they are too big to fit through the birth canal. At the same time, since the genes that affect the size of the human pelvis come from both men and women and the researchers think there may be advantages to not developing ever-wider hips, that space hasn’t become constantly wider to support larger babies.
The modern change that they predict is that C-sections make it easier to pass on the genes that would account for a small birth canal, potentially increasing rates of these potentially dangerous births.
According to a model the researchers created, they predict that C-sections may have caused these cases of obstructed labour to become 10 to 20% more common since roughly the 1950s, going from affecting about 30 out of every 1,000 births to affecting 33-36 out of every 1,000 births. (They don’t actually know whether these rates have gone up, but they expect that if their model is correct, they have.)
“Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection,” Philipp Mitteroecker, a theoretical biologist at the University of Vienna and lead author of the study, told the BBC.
But of course, showing evolutionary changes in a short time span — about 60 years — is very difficult. Most evolutionary change occurs on scales of thousands or even millions of years, though in small populations it can happen more quickly. In addition, it’s hard to know if obstructed labour is in fact becoming more common at all, and if so, it’s even harder to know if that is strictly the result of C-sections.
What the data shows
C-sections have been a part of the human birth process for thousands of years. Greek mythology, ancient Hindu and Egyptian texts, Chinese etchings, and Roman history includes references to these types of births. But the modern era of safe and common C-sections began much more recently, after the development of antibiotics and after births in hospitals became the norm.
But the majority of C-sections do not occur because of obstructed labour, meaning that a rise in the number of C-sections doesn’t necessarily mean these rates have gone up.
If obstructed labour occurs in 3% of births (or, if rates have increased, 3.3-3.6%), that still only accounts for a very small percentage of all of the C-sections performed. In the US, close to a third of all births happen by C-section. (Researchers say that ideally, these rates should be closer to 19%.) Many C-sections are the result of medical necessity caused by factors like obesity and diabetes. Others have to do with obstetrics ward policies or — perhaps — financial incentive or fear of malpractice lawsuits. In other words, obstructed labour is not the main factor here.
So are C-sections changing evolution? Maybe.
The researchers here simply say that their model can explain why obstructed labour persists as part of the human condition and they say it predicts how C-sections could make this more common.
It’s certainly possible, likely, that a change in the way large numbers of people give birth has some effect on humanity.
“It would never occur to me that cesarean sections would not have an effect,” biological anthropologist Karen Rosenberg tells The Daily Beast. “The idea that human behaviour affects our evolution is a central idea in understanding of evolution. All kinds of things — when we cook our food, when we share food, when we build shelters — everything we do as cultural animals has the potential to affect our biology.”
But for now, we don’t even know that obstructed labour is becoming more common. There’s probably some effect or multiple effects that C-sections are having on humanity, but without more data, we can’t specifically say what they are.
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