Caesarean births ‘affecting human evolution’

Dec 6, 2016

By Helen Briggs

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.

Dr Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna, said there was a long standing question in the understanding of human evolution.
“Why is the rate of birth problems, in particular what we call fetopelvic disproportion – basically that the baby doesn’t fit through the maternal birth canal – why is this rate so high?” he said.

“Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection. “Women with a very narrow pelvis would not have survived birth 100 years ago. They do now and pass on their genes encoding for a narrow pelvis to their daughters.”


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10 comments on “Caesarean births ‘affecting human evolution’

  • From Wikipedia
    https://en.wikipedia.org/wiki/Pelvis

    Variation[edit]
    See also: Sex differences in humans
    Modern humans are to a large extent characterized by bipedal locomotion and large brains. Because the pelvis is vital to both locomotion and childbirth, natural selection has been confronted by two conflicting demands: a wide birth canal and locomotion efficiency, a conflict referred to as the “obstetrical dilemma”. The female pelvis, or gynecoid pelvis,[7] has evolved to its maximum width for childbirth — a wider pelvis would make women unable to walk. In contrast, human male pelvises are not constrained by the need to give birth and therefore are more optimized for bipedal locomotion.[8]

    The principal differences between male and female true and false pelvis include:

    The female pelvis is larger and broader than the male pelvis which is taller, narrower, and more compact.[9]
    The female inlet is larger and oval in shape, while the male sacral promontory projects further (i.e. the male inlet is more heart-shaped).[9]
    The sides of the male pelvis converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart.[10]
    The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90-100 degrees) in women. Accordingly, the angle is called subpubic angle in men and pubic arch in women.[9] Additionally, the bones forming the angle/arch are more concave in females but straight in males.[11]
    The distance between the ischia bones is small in males, making the outlet narrow, but large in females, who have a relatively large outlet. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males. The greater sciatic notch is wider in females.[11]
    The iliac crests are higher and more pronounced in males, making the male false pelvis deeper and more narrow than in females.[11]
    The male sacrum is long, narrow, more straight, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.[11]
    The acetabula are wider apart in females than in males.[11] In males, the acetabulum faces more laterally, while it faces more anteriorly in females. Consequently, when males walk the leg can move forwards and backwards in a single plane. In females, the leg must swing forward and inward, from where the pivoting head of the femur moves the leg back in another plane. This change in the angle of the femoral head gives the female gait its characteristic (i.e. swinging of hips).[12]

    Check out this diagram that compares the pelves/fetuses of P. troglodytes, A. afarensis, and H. sapiens.

    http://d1vn86fw4xmcz1.cloudfront.net/content/royptb/370/1663/20140063/F2.large.jpg

    From:
    http://rstb.royalsocietypublishing.org/content/370/1663/20140063

    Having birthed three nine pounders, I envy the pelvic “wiggle room” of my fellow non sapiens primates. Imagine how frightening the imminent prospect of labor and delivery must have been in the past when there was very little that medical staff could do to help in the event of cephalopelvic disproportion.



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  • Odd to limit the discussion to C-sections, though. All of medicine “overrides” selection processes. Our NICU saves extremely premature babies. Antibiotics save countless millions every year. Trauma surgeons save victims of knife and gun fights. and on and on. Social programs both save people who might have starved or frozen to death, and perhaps encourage greater fecundity. greater education results in lower reproductive rates. But speaking of the c/s rate: Tiny Asian ladies marry NBA basketball players and have babies too large for their pelvis. Fast foods and obesity have resulted in the average baby weighing a pound more than a decade ago- without any change in the size of the birth canal. Fetal monitoring results in more frequent C-sections- and, we hope, saves babies who otherwise would have never lived to the age of reproduction.

    Practically every aspect of modern society affects survival/reproduction in ways that alter the definition of reproductive fitness for humans.



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  • @Brian, huh?
    who is limiting the discussion? I’ll grant every point you make, but it’s a story about a study of the effect of caesarean births. Perfectly reasonable to keep the talk to the evidence being presented.



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  • There is a focus on a narrow pelvis in this article. But isn’t it possible that baby’s heads have become larger? I think the same evolutionary logic can be applied here. In fact it seems more logical to me. Partner choice and intelligence are probably linked. A big head means on average more intelligence. So there is a positive evolutionary pressure for big heads, I think. An evolutionary pressure for small pelvises does not seem as likely to me.



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  • hfaber #5
    Dec 6, 2016 at 4:21 pm

    There is a focus on a narrow pelvis in this article. But isn’t it possible that baby’s heads have become larger? I think the same evolutionary logic can be applied here. In fact it seems more logical to me. Partner choice and intelligence are probably linked. A big head means on average more intelligence. So there is a positive evolutionary pressure for big heads, I think. An evolutionary pressure for small pelvises does not seem as likely to me.

    These competing selection pressures are explained on the BBC link.

    ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬ ▬

    @BBC link – Opposing forces

    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 in their theoretical study.

    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.

    “One side of this selective force – namely the trend towards smaller babies – has vanished due to Caesarean sections,” said Dr Mitteroecker.

    “Our intent is not to criticise medical intervention,” he said. “But it’s had an evolutionary effect.



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  • Just point out that this all appears a little arrogant coming from the West. I’m sure it is having this impact in the West, but there are many millions who still starve to death every year, millions in poverty who die still during childbirth without the benefits of modern medicine. All of these people are being selected for or against by natural selection. I don’t know what level of immigration we need to accept to reduce the our modern impacts on our western genomes but I image it wouldn’t take to many refugees to strengthen our gene pool again (of course that is if we didn’t insist as my country to my shame is – in locking them up in poor surrounding nations). Our empathy and social welfare systems will need to get one hell of a lot better I think before this becomes a real issue globally.



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  • 30 to 36 in a thousand in a fifty year period is not massively significant is it? I think the headline is slightly overblown / clickbait.
    The rise in caesareans (in the west at least) in mainly due to women being older mothers and therefore more likely to have complications and therefore doctors often go down the caesarean route as its safer on average for this age group (38+). This was the case with my daughter who was at huge risk of being breech.
    Rising obesity is also a reason for more caesareans.



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  • @ MrDNA,

    Yes, C’ sections are a bit of a political issue as well.

    The midwifes at the hospital where my son was delivered were excellent in many ways but were also peppered with woo in others (for example- they promoted a variety of natural remedies). It made it difficult for us to make decisions about which way to act they outlined the benefits of natural birth (which I do not doubt are real) but it was clear that they had issues with doctors over using it. I wanted the numbers. It was looking like my son was going to be breech and our hospital does not do breech births if they can avoid it they will scheduled a C section. So we needed to make a decision, the midwifes where encouraging us to try different things from some woo candle thing (burning smelly candles on your feet) they claimed there had been studies supporting the effectiveness of this so I found the paper read it found it was a small study which did indeed find there was some success in turning babies with this method it appears whatever was in the candles was inducing massive abdominal contractions which in some patients succeeded in turning the baby but in two cases (there were 12 or so in the trial so not a big study) resulted in detached placates and emergency trips to the hospital to get an emergency c section others stopped the treatment because of the pain and the trail was canceled at that point. The midwives were surprised when it handed them over the evidence of what they’d be advocating.

    They then suggested to my wife that we could try to get the baby turned with physical manipulation. I asked the question of what could go wrong and how often does it go wrong so we could have some basis on which to make a judgement. They knew exactly what could go wrong (detached placenta again) and in which case they would perform an emergency C section. Okay so how often does this happen? That they could not tell me, I pushed for them to find out and eventually one of the doctors told me about 1 in 200. That was enough for me and my wife and we singed in for a c section the next day.

    My wife would have preferred the natural child birth at the time and she felt guilty afterwards for not going through this right of passage. For me though its all about the relative risks all going well it would appear that natural birth is better but given safety of certain complications it’s unlikely we are going to be mandating natural child birth.



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