The number of babies born by Caesarean section has affected Birth evolution, a study has suggested.

As experts say that the increasing use of Caesarean sections is having an effect on human evolution, Israel boasts the lowest rate of such births in the OECD.

More mothers now need surgery to deliver a baby due to their narrow pelvis size, with researchers saying cases where the birth canal is too narrow have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today, BBC News reports.

This trend, say Austrian researchers, is likely to continue but not to the point that non-surgical births will disappear.

With an average rate of 154.3 caesarean sections per 1,000 births, Israel has the lowest prevalence of such procedures out of 34 OECD countries – while also having the highest overall birth rate, the Jerusalem Post reports. The OECD average of caesarean sections is 258.7 per 1,000 births.

Researchers suggest that the global rise in caesarean sections may be the result of evolution: as medical intervention has made birth problems far less fatal, women with very narrow pelvises are surviving childbirth and passing on their genes, unlike in the past.

Dr. Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna, told BBC News: “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.”

Researchers studying the phenomenon, who gathered data from the World Health Organization and other large birth studies, found opposing evolutionary forces.

They found that on one hand, there is a trend towards larger newborns, which are healthier, but also causes them to get stuck during labor.

If not for modern medical intervention, this situation would have proved lethal for both mother and baby. In addition, it would stop their genes from being passed on.

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

“The pressing question is what’s going to happen in the future?” he added. “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.”