Caesarean sections that help difficult births – such as when a baby cannot fit through its mother’s pelvis – are affecting human evolution, according to a new study. But do the claims stack up?
What’s being said?
Some babies are too big to fit through their mother’s pelvis and would once have died in childbirth, sometimes killing their mums too. It may be down to the “obstetrical dilemma”. Bigger babies are more likely to survive childhood; yet there are also pressures to keep women’s pelvises narrow, not just because it might aid upright walking, but also because it reduces premature births.
Like most features of human anatomy, size at birth and pelvis width are highly variable, with their distribution in the population following bell-shaped curves. The same goes for the ease of a baby’s “fit” through the pelvis. With evolutionary pressures forcing the fit to be as tight as possible, there is always going to be some fraction of babies – on one side of the curve’s tail – that can’t squeeze through. Thanks to C-sections, these babies no longer die, so this fraction should be rising – by up to 20 per cent since the procedure was introduced, according to Philipp Mitteroecker of the University of Vienna, Austria.
How could evolution be happening so fast – doesn’t it take millennia?
Not necessarily – evolution can happen over a single generation if the environmental changes are drastic enough. For instance, think of a new plague that wipes out nearly all of one animal species on an island, leaving behind only those who happen to be naturally immune. That’s evolution happening in one generation. In the case of C-sections, we may have had two or three human generations since they became more common in the 1950s.
How do we know if it’s happening?
Mitteroecker’s team hasn’t produced any evidence that it is. The study was theoretical work, based on plugging observed figures for the rate of obstructed childbirth into their models. Their predicted rise of 20 per cent is still quite small, though – it would have taken the rate of obstructed births from 3 per cent, a conservative estimate for the rate before C-sections, up to 3.6 per cent today.
Haven’t C-sections risen by more than that anyway?
Yes, the C-section rate is now about 25 per cent in the UK. So even if evolution is having an effect, it would be hard to disentangle from all the other factors driving the rise in C-sections.
That depends on who you ask. Natural childbirth proponents say it’s because doctors insist on medicalising the birth process. Doctors on the other hand, say rates are rising because women are having babies later in life, when their body’s tissues are less stretchy. Mothers are also fatter today, and are more likely to have medical conditions such as diabetes, which all make birth more difficult. Not to mention as a society, we’re also less willing to accept any degree of risk.
And aren’t women just too posh to push these days?
Actually, some doctors argue there’s a reasonable medical case for avoiding a vaginal birth, which carries a 90 per cent risk of a tear or cut to the genitals and over a 5 per cent risk of tearing through the anus in a woman’s first birth. In the UK, women are now supposed to be allowed to choose C-sections without medical reasons, as long as they are told about the risks of surgery, which include infection and blood loss. There’s also concern that C-section babies miss out on good bacteria from the mum’s vagina, so some women swab their baby’s face with their vaginal fluid – although we don’t know yet if that’s a good idea.
So how will we know if evolution really is affecting the C-section rate?
We won’t. But Mitteroecker says the idea is something to consider in discussions about the “right” level of C-sections, because most assume the number of obstructed births is constant. And natural childbirth proponents who urge women to “trust birth” might also want to bear it in mind.