Human Evolution: How Cesarean Deliveries Are Changing Women's Bodies

Fewer babies can fit through their mother's birth canal in part because the growing use of Cesarean sections means more women have a narrow pelvis size. In the past, those mothers and daughters would have died during the delivery process, making it less likely for the genes allowing for a narrow pelvis size to be passed on, BBC News reported Tuesday.

The number of cases where a baby was too big to pass through the mother's birth canal soared from 30 in 1,000 in the 1960s to 36 in 1,000 births. The trend suggests Cesarean sections are changing human evolution, said Philipp Mitteroecker, of the department of theoretical biology at the University of Vienna.

"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."

Roughly 32 percent of all deliveries in the U.S. are done by Cesarean, according to the Centers for Disease Control and Prevention. In all, there were 2,699,951 vaginal deliveries and 1,284,551 Cesarean deliveries in 2014.

Cesarean surgeries were once only used to save a dying mother, but as they've become more common around the world in recent years, the widespread use has sparked a debate over whether the procedure is necessary versus optional. Researchers have sought to link vaginal delivery to healthy immune systems, suggesting an infant can pick up useful bacteria from a mother's birth canal.

"Our intent is not to criticize medical intervention," Mitteroecker said. "But it's had an evolutionary effect. "