Caesarean births 'affecting human evolution'

[nitpicker]

http://www.bbc.com/news/science-environment-38210837

Caesarean births 'affecting human evolution'

By Helen Briggs
BBC News

5 December 2016

From the section Science & Environment

The regular use of Caesarean sections is having an impact on human evolution, say scientists. More mothers now need surgery to deliver a baby due to their narrow pelvis size, according to a study.

Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today. Historically, these genes would not have been passed from mother to child as both would have died in labour.

Researchers in Austria say the trend is likely to continue, but not to the extent that non-surgical births will become obsolete.
(snip)

It has been a long standing evolutionary question why the human pelvis has not grown wider over the years. The head of a human baby is large compared with other primates, meaning animals such as chimps can give birth relatively easily.

The researchers devised a mathematical model using data from the World Health Organization and other large birth studies. They found opposing evolutionary forces. One is a trend towards larger newborns, which are more healthy. However, if they grow too large, they get stuck during labour, which historically would have proved disastrous for mother and baby, and their genes would not be passed on.
(snip)

[hlthe2b]

1. Umm, Obesity is a well documented reason for at least some of the increase in c-sections

Being overweight or obese, respectively, was associated with adjusted 39% and 2.9-fold increased likelihood of cesarean delivery during the first stage of labor, compared with normal-weight women, but no increase in second-stage cesarean delivery (Obstet. Gynecol.

Given the incredible increase in obesity rates world wide and seemingly non-ending trend, I think that needs to be the focus.

[enough]

2. Isn't that rather fast for such a significant evolutionary change? (nt)

[BainsBane]

3. Exactly

I call bullshit.

[Nitram]

4. I don't think it's B.S. to suggest that a trend like this could powerfully influence evolution...

... in the long run. The size of the baby's head in relation to the width of the female pelvis has always been an evolutionary limit on both factors and a powerful selective pressure until birth by Ceaserian section became routine. This is a good example of classic evolutionary theory in action. Babies and/or mothers that should have died during the birth process will now survive and the genes for smaller hips and/or larger heads will not be eliminated from the population. Don't make the mistake of assuming this means that all babies will soon have larger heads and all mothers will a narrower pelvis. Rather, it could very well lead to a statistically significant change in the number of those mother and their babies in the population that fit one or both of those categories.

[BainsBane]

5. Yes, but could influence is different from

Has already influenced.

[Nitram]

6. It is possible it has already been influenced as evidence noted by the article suggests.

"Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today." I don't know if that's statistically significant, but it does suggest the population may already be experiencing the effects of an evolutionary trend.

[Gothmog]

7. Interesting article

[Warpy]

8. That and the current dumb male preference for waiflike women

with very narrow hips.

Still, a lot of those c-sections are done unnecessarily in first world countries.

[BadgerKid]

9. Sub-1%, eh. Not very impressive. n/t

[Bearware]

10. I wonder if it is vaguely possible another variable might be involved?

Association between vitamin D deficiency and primary cesarean section.
https://www.ncbi.nlm.nih.gov/pubmed/19106272

Or perhaps we are evolving a fear of exposure to direct sunlight.