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 an Austrian study.
However the findings have come under criticism for having a small sample size and over-emphasising the rate of evolution.
The researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1000 in the 1960s to 36 in 1000 births today
Historically, these genes would not have been passed from mother to child as both would have died in labour.
The researchers 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 longstanding 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.”
The research highlights a question that has puzzled scientists for decades: 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 Organisation and other large birth studies.
They found opposing evolutionary forces in their theoretical study.
One is a trend towards larger newborns, which are healthier.
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.”
The researchers estimate the global rate of cases where the baby could not fit through the maternal birth canal has increased 10-20% over the past 50 or 60 years.
“The pressing question is what’s going to happen in the future?” Dr Mitteroecker said.
“I expect that this evolutionary trend will continue but perhaps only slightly and slowly.
“There are limits to that. So I don’t expect that one day the majority of children will have to be born by [Caesarean] sections.”
The research is published in the journal, Proceedings of the National Academy of Sciences.
Commenting on the study, Daghni Rajasingam, a British consultant obstetrician and a spokesman for the Royal College of Obstetricians, said other factors, such as diabetes and obesity, are having an impact on the number of caesarean sections.
“I think what is important to take into the [question of] evolution is that things like diabetes are much more common at a younger age so we see many more women of reproductive age who have diabetes,” she said.
“That has consequences as to whether or not they may need a caesarean section.
“In addition, the rates of obesity are increasing so more and more women of reproductive age have a higher body mass index and this again has an impact on caesarean section rates.”
In NEW ZEALAND, a major study showed 35% of babies were delivered by Caesarean in National Women’s at Auckland City Hospital in 2013.
That report showed the rate had risen every year since 2009. In 1998, it was 25%.
In the Counties Manukau District Health Board area, where the rate had traditionally been relatively low, it was up from 14% in 2003 to 23% in 2013.
At National Women’s, 17% of births were “elective” caesareans and 18% were emergency caesareans. Electives include those planned because of risks to the baby, and an unknown proportion – no one has done the research to find out – that are thought likely to be medically unnecessary, such as the “too posh to push” mothers.
In NEW SOUTH WALES, a landmark policy introduced in 2010 aimed to lift the vaginal birth rate to 80% by 2015. In fact the state’s vaginal birth rate dropped by 1.7% to 56%.
The rate of elective caesareans increased (17.7% to 19.8%) while the rate of emergency caesareans did not budge.
The rate of vaginal births has remained stable at Sydney’s largest public hospitals, compared to the relentless downward march in private facilities.