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More and more women must undergo a cesarean because they have a pelvis too small to give birth by natural birth.
And, the regular practice of this surgery is having an impact on human evolution, according to a new study published in the journal Proceedings of the National Academy of Sciences.
According to the researchers, cases in which the baby does not fit in the birth canal have increased from 30 in 1000 in the 60s to 36 in 1000 today.
In the past, these genes would not have passed from mother to daughter since both of them, in both cases, would have died in childbirth.
Austrian scientists believe that this trend may continue but not to the point that natural births become obsolete.
“Without modern medical interventions this kind of problems used to be lethal, and this, from the point of view of evolution, is natural selection,” said Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna.
“Women with a narrow pelvis would not have survived 100 years ago, and now they pass on their genes that encode information for a narrow pelvis to their daughters.”
Scientists have long wondered why the human pelvis has not widened over the years.
The head of a human baby is large if we compare it with that of other primates, which makes delivery much more difficult than, for example, for chimpanzees.
The researchers designed a mathematical model using data from the World Health Organization and other powerful studies on births.
What they found were opposite evolutionary forces.
On the one hand, there is a tendency for larger newborns , who are healthier.
However, if they grow too much, they get stuck during childbirth, which, in the past, was fatal for the mother and the baby, and the mother’s genes, therefore, did not pass to the next generation.
“One side of this selective force, let’s call it the tendency to have smaller babies, is disappearing because of cesareans,” explains Mitteroecke.
“Our intention is not to criticize the medical intervention”, clarifies the expert, but to show “that it has had an evolutionary effect”.