Opting for a caesarean is becoming increasingly popular but a new study has found that choosing this option is affecting human evolution in terms of how big babies are becoming.

C-sections are widening the gap between the small size of female pelvises and the size of newborns – almost a 10 to 20 percent gap to be precise, according to a new study led by Philipp Mitteroecker, an assistant professor with the Department of Theoretical Biology at the University of Vienna, Austria.

While the average woman’s pelvis has remained small, the size of newborn babies have increased leading to more C-sections being performed.

“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.”

Caesarean births have increased over the past decades with one in three women opting to have a C-section either as their birthing plan or during labor.

While babies seem to be getting bigger, will this human evolution one day see every birth being a C-section? Well Mitteroecker says not likely.

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

Details of the study were published in the journal Proceedings of the National Academy of Sciences.