Evolution is typically thought of as a slow process, happening over millennia—but that's not always the case. A new study suggests that the success of Caesarian births in the second half of the 20th century could be influencing the evolution of human populations.

Since World War II, Caesarian births (also known as C-sections) have been on the rise thanks to surgical advancements, making them safer and cheaper. While once considered only an emergency option, that is no longer the case. These days roughly a quarter of new mothers in the United Kingdom and around one-third of mothers in the United States give birth though C-section, Peter Walker reports for The Independent. But now a group of researchers from the University of Vienna believe that the procedure’s popularity could be starting to alter the course of human evolution.

Their study, recently published in the Proceedings of the Natural Academy of Sciences, suggests that an increasingly common reason women undergo C-sections is because their pelvis is too narrow. And, according to the study, the number of babies too large to fit is up by 20 percent since C-sections began.

These “obstructed births” are one of many complications that this surgery was designed to assist, but this could also mean that the genes for bigger babies are being passed down, theoretical biologist and lead author of the study Philipp Mitteroecker tells Helen Briggs for the BBC. In the past, however, the condition often proved deadly, meaning that these genes were not passed down through the generations.

"Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection,” Mitteroecker tells Briggs. "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 study, however, is preliminary and we can't know for sure if this evolutionary change is indeed taking place. As Clare Wilson writes for The New Scientist, "Mitteroecker’s team hasn’t produced any evidence that it is [happening]. The study was theoretical work, based on plugging observed figures for the rate of obstructed childbirth into their models."

The rise the team predicted is small—from about three percent to roughly 3.6 today. And there are many other factors that can complicate these conclusions. Many women are having babies later in life, for example, which means they’re giving birth when their bodies are a little less pliable. Weight and other health issues can also play a part in whether a doctor recommends a C-section.

"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,” Daghni Rajasingam, a consultant obstetrician and a spokesman for the Royal College of Obstetricians, tells Briggs. "That has consequences as to whether or not they may need a Caesarean section.”

This question of whether or not C-sections are driving evolution isn't a judgment of the procedure, considering the many lives that have been saved by these surgeries. But is important in understanding the history of how our species came to be.