Regular use of Caesarean sections is having an impact on human evolution, say scientists in a new study, as more and more mothers require surgery to deliver their baby due to narrow pelvis size.
Researchers estimate that 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.
Scientists point out that, historically, the “narrow pelvis” genes would not have been passed on, as both mother and child would have died in labor.
“Without modern medical intervention, such problems were often lethal and this is, from an evolutionary perspective, natural selection,” said Dr. Phillip Mitteroecker, of the University of Vienna.
“Women with a very narrow pelvis would not have survived birth 100 years ago. They do now, and pass on their genes for a narrow pelvis to their daughters.”
Scientists say that while larger newborns are healthier and, back in prehistoric times, would have survived longer, if they were too large they would get stuck during labor and their genes would not be passed on.
But, with the rise of C-section births, larger babies are born and pass on their genes, so (theoretically) babies get bigger and bigger over time.
Over the past 50 or 60 years, there’s been a 10-20% increase in cases where the baby could not fit through the birth canal.
“The trend towards smaller babies has vanished due to Caesarean sections,” explained Dr. Mitteroecker, “Our intent is not to criticize medical intervention, but it’s had an evolutionary effect.”
“The pressing question is, what’s going to happen in the future? I expect that this evolutionary trend will continue,” added Mitteroecker.