Risk of Caesarean section is heritable—Natural selection cannot reduce the rates of obstructed labour

October 16, 2017, University of Vienna
Biologist Philipp Mitteroecker found that women born by Caesarean section due to a fetopelvic disproportion (FDP) are more than twice as likely to develop FDP when giving birth than women born naturally. Credit: University of Vienna

During the last decades, rates of Caesarean section have multiplied; by now it is one of the most frequently performed surgical treatments worldwide. Even if many of these C-sections are not strictly medically indicated, human childbirth is complicated and risky compared to that in other primates. Why has evolution by natural selection not led to a wider birth canal, thus reducing the high rates of obstructed labor?

Recently, a team headed by Philipp Mitteroecker, an at the University of Vienna, showed that these are a direct consequence of the distinct characteristics of obstetric selection in humans.

Using a mathematical model, the so-called "cliff edge model", they showed that cannot reduce the rates of obstructed labor. In fact, only weak selection for a large neonate, a narrow pelvic canal, or both is sufficient to account for the considerable incidence of fetopelvic disproportion (FPD, i.e., the fetus is too large to pass the maternal ). Based on this model, they also predicted that the regular use of life-saving Caesarean sections throughout the last decades has already triggered an evolutionary increase of fetopelvic disproportion rates by 10 to 20 % (which equals roughly half a percentage point of the initial incidence).

In the recent study, Mitteroecker and colleagues use the cliff edge model to predict that women born by Caesarean because of FPD are 2.8 times more likely to develop FPD in their own childbirth compared with women born vaginally. This strong effect should be easier to detect in epidemiological data than the evolutionary increase predicted.

"Indeed", says Mitteroecker, "we found empirical studies reporting risks for Caesarean up to twice as high for women born by Caesarean as compared to women born vaginally." This theoretical prediction of a complex epidemiological pattern lends support to the cliff edge model and its evolutionary implications.

The study is published in PNAS.

Explore further: Researchers suggest increased instances of C-sections are causing evolutionary changes

More information: Philipp Mitteroecker el al., "Cliff-edge model predicts intergenerational predisposition to dystocia and Caesarean delivery," PNAS (2017). www.pnas.org/cgi/doi/10.1073/pnas.1712203114

Philipp Mitteroecker et al, Cliff-edge model of obstetric selection in humans, Proceedings of the National Academy of Sciences (2016). DOI: 10.1073/pnas.1612410113

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5 / 5 (2) Oct 16, 2017
This is why genetic engineering is necessary and inevitable.
Whydening Gyre
not rated yet Oct 16, 2017
This is why genetic engineering is necessary and inevitable.

Ok, then...
Do you engineer for smaller babies or larger vaginas?
not rated yet Oct 16, 2017
Ok, then...
Do you engineer for smaller babies or larger vaginas?

neither. you filter out the variations in the gene pool that don't fit
Da Schneib
1 / 5 (1) Oct 16, 2017
The reason it's not genetically filtered for is because the woman generally dies and the baby generally lives if the C-section is not performed when indicated. Rather nasty if one thinks about it. This FPD is a major risk in childbirth.

I note that this article seems to avoid the issue. I might bother to read the paper to see if the authors are more blunt. But I kinda suspect not.
Da Schneib
1 / 5 (1) Oct 16, 2017
Had a look. Yep, the existence of C-section has reduced the selection for FPD; and yep, this paper doesn't directly mention it. FPD is pretty obvious, it's discernible from even X-rays, and even moreso from other 3D imaging techniques. I'm not saying it's bad to do C-sections, or bad to preserve the life of the mother; I'm saying that since this is a problem genetics has created, we have to decide how we'll deal with it and genetics may not be the right way. Engineering for smaller skulls may turn out to be engineering for stupid people. Engineering for larger birth canals may not work out any better. Beware of the Law of Unintended Consequences. We already got enough stupids as a brief tour of this site will confirm for any reasonable observer. If I was a woman with slim hips I might choose to protest a law requiring broad hips in order to have babies. Unintended Consequences can occur in multiple dimensions.
3 / 5 (2) Oct 17, 2017
This is why genetic engineering is necessary and inevitable.

Or artificial wombs.
not rated yet Oct 17, 2017
C-section has been a way to do 'unnatural' selection (note that 'unnatural' is not the same thing as 'wrong' in this case).
Selection for genetic traits is only relevant to traits that are expressed until childbirth (for women) or insemination (for men).
Anything that comes after that is not selected for or against directly (There are indirect things like a hormone levels that results in a more caring attitude which does play some part in selection afterwards. But they are not as strong as the 'do or die' traits that select before)

C-section takes a factor out of the childbirth process and thus makes it a trait that just depends on natural variability rather than on actual selection. Seems logical to me that this would normalize the number of women with FDP as opposed to earlier where this was heavily selected against.

(Not to mince words: "Selection" mostly means "premature death" - at the very least inability to conceive living children.)
not rated yet Oct 17, 2017
Beware of the Law of Unintended Consequences.
we already have unintended consequences. we introduced artificial selection and not a beneficial one. for all we know this will lead to more consequences than we presently realize and its spreading.
If I was a woman with slim hips I might choose to protest a law requiring broad hips in order to have babies.
that's dark and extreme. genetic counseling has already proven its effectiveness and no one is forcing people to get it, they choose to. if you were a woman knowing you can only have a child by C-section and will likely pass it on to your child wouldn't you want the option to spare them that hardship? a lot of people do.
1 / 5 (1) Oct 17, 2017
I just wish we were hearing from women about these issues.

Cause the women I have known find it rather tedious, that men talk at them about female issues but never listen to what the women are saying.

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