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HEAD FOR THE EXIT

Scientists are predicting that c-sections will lead to bigger babies

By Lila MacLellan

Since the world’s first documented case of a woman surviving a cesarean section in medieval Prague, the cesarean section has become a catch-all solution for all manner of problems surrounding a child’s birth.

Heavy reliance—or overreliance—on the procedure has even led an evolutionary biologist at the University of Vienna to hypothesize that, after only two generations of routine use, c-sections are influencing the way humans are evolving. The scientist, Philipp Mitteröcker, proposed that the procedure will continue to alter the course of human evolution in a new paper in the Proceedings of the Academy of Science.

The c-section, as we know the procedure today, was not advanced enough to be reliably safe until the middle of the 20th century. Since the 1970s, it has become increasingly popular in developed countries: Forty years ago, only 7% of U.S. births were performed by c-section, but today that figure stands at 30%, with rates in the UK and other wealthy nations trailing closely behind.

C-sections were initially developed to solve the problem of fetopelvic disproportion, the potentially fatal scenario when a baby is too large for the mother’s birth canal. It’s these c-sections that interest Mitteröcker, who argues that the surgery has allowed the genes for smaller hips in mothers and babies and bigger babies to survive and propagate rather than be selected out by the evolutionary process. Before c-sections were standardized, women with narrow hips often died during labor.

By examining research from the World Health Organization and major birth studies, Mitteröcker’s team found that cases of obstructed pregnancies have increased from a global rate of 30 per 1,000 a few decades ago to 36 per 1,000 today. “One side of this selective force—namely the trend towards smaller babies—has vanished due to caesarean sections,” Mitteröcker told the BBC. He made clear that he was not criticizing medical intervention, but pointing out that it has an effect on evolution.

He believes that with continued use of c-sections, neonate bodies and especially heads will continue to get larger (the better to contain bigger brains) while the hips of mothers and babies will continue to stay small (which is thought to optimize walking and standing functions), but only to a point, because mothers have a limited capacity to produce energy for themselves and a child in utero. As a result, he doesn’t believe medically necessary c-sections due to fetopelvic disproportion will overtake natural vaginal births.

Mitteröcker has said that his team is pursuing a longer line of generational birth data, along with hereditary details and skeleton size records, to prove this theory, while limiting the studies of c-sections to data involving life-or-death interventions. (Non-critical c-sections aren’t considered likely to play a role in evolution, because in those cases the mothers would likely have survived a vaginal birth.) Other experts have noted that factors like rising rates of  maternal obesity, which tends to be associated with higher childbirth sizes, complicate efforts to pinpoint what is driving higher rates of c-sections and bigger babies.