Tuesday, 6 December 2016

Caesareans May Be Altering Human Evolution. But Not Much, And I Don't Care.



According to reports on the BBC (and elsewhere) today, the number of women having C sections is causing our species to evolve bigger heads and / or smaller pelvises and so we are becoming less able to give birth without surgical help. 

The Miami Herald got particularly upset declaring: 

"C-Sections Are Increasing Because We're Messing With Evolution..."

I wrote last year about a similar idea from Obstetrician and (IMHO) slightly questionable old chap, Michael Odent. His opinion was based on, well, his opinion but these new stories actually come from a real life proper scientific journal. This month's PNAS.

So are the headlines all true this time?

Well, yes, maybe and no.

It all sounds very dramatic in the news reports, the idea that we are messing with nature and pushing evolution to make us ever more dependent on modern technology. It has a grippingly dystopian feel. It is true that women and babies now survive where once their respective small pelvises or large heads would have fatally removed them from our gene pool. I am one of those women. My genes only made it to my daughter and to her younger sister because cold hard modern medicine overruled nature.

So it is possible that children like mine will go on to need surgical births themselves, where in the past they would simply have never lived to give birth at all. With a few more women like us around the human population as a whole may need more C sections in future. We may evolve to, on average, do birth a little bit different.

But that's not in any way special.


Almost everything we do that keeps babies alive impacts on our evolution. Genes that confer all manner of slight disadvantage can make it from generation to generation because everything from clean water and plentiful food to vaccinations and childhood heart surgery mean that children who would once have died are living long enough to keep those genes in circulation. There may be a few more people around with heart conditions, or a tendency to get food poisoning but we assume that if we can treat those problems now, then we will continue to do so, and probably better, in the future. 

The need for a surgical birth is no different, and it is also a very very small change. 

Read on a bit beyond the scary sounding headlines and the photos of distressed looking mothers and it becomes clear that we are talking about a fairly small number births. The study estimates that the number of C sections for obstruction (where the baby's head is too big to fit through the mother's pelvis) was around 30 per 1000 births in the 1960's and has now "evolved" to be roughly 36 per 1000. A tiny proportion of the total C section rate of around 250-300 or more per 1000 in many developed countries. The evolutionary effect of women like me surviving childbirth isn't a big driver in the increase in CS rates, there are plenty of other culprits to look for there.

Messin' with the gene pool


But even that small influence may be overstated by the research. Genetics isn't as simple as - you get what your mum had. There are multiple genes and environmental factors at work. Dad's also have something to do with it. A small headed woman, with a normal pelvis, and a mother who popped babies out with no problems, could find herself in need of a C section because her baby's father passed on a whole bunch of big head genes (looking at no one in particular Mr SB...). Probably more significantly, a lot of women with perfectly average genes produce overly large babies because of factors that are becoming far more common such obesity or gestational diabetes or just through plain old chance. 

Big babies are usually an advantage. Evolution doesn't make perfect finished products, it goes for the best possible compromise and big heads with their big brains are so useful to the species as a whole that nature is prepared to accept a few individual casualties where things go too far. So even without any C section caused evolution, big babies will continue to happen.

Even very big babies can often be born vaginally if they are in the right position. Heads squish and pelvises stretch. My first child was a stonking 9lb 10oz (about 4.5kg) but plenty of people have babies that big. Perhaps my pelvis was too small for her or, more likely, the problem was her difficult position. Babies heads aren't perfect spheres and pelvises aren't round holes, try to stick one through the other the wrong way around and it just doesn't work (See this blog about the study for a lot more info on that). 

So, as is so often the case, what we have here is some academically interesting work about something that isn't very significant for the vast majority of people but which has, by virtue of being about women's bodies, been built into a futuristic horror story. Something to both chastise us for our supposed choices and terrify us about our most powerful, normal, female act. 

There may be a slight evolutionary shift in how we give birth because of the availability of a safe way to save some of us from death in childbirth but it won't spell the end for normal human reproduction. I for one will be sorry if I have passed on genes to my daughters that will make it harder for them should they ever choose to become mothers. I hope the many other factors at work will spare them surgery. I would myself have preferred normal births to the blood loss and grueling recoveries of my c sections. But without those c sections I would not be here, my daughters would not be here and if the price of their lives, and all the other babies like them, is yet another tiny tweek in our constantly tweeked gene pool then tweek away I don't care, bring on the evolution.

SBx

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