Cesarean sections appear to do nothing but ease childbirth but it looks like the procedure may also affect human evolution.

A study published in the Proceedings of the National Academy of Sciences explored the long-standing evolutionary question why women's pelvis have not evolved to be wider to more easily accommodate fetus size, especially now that babies are being born larger.

According to Philipp Mitteroecker, from the department of theoretical biology at the University of Vienna, and colleagues, routine use of C-sections has reduced the rate of maternal deaths but has increased the rate of fetopelvic disproportion, or when a baby doesn't fit through their mother's birth canal.

C-Sections And Human Evolution

Larger babies are healthier babies so the prevalence of bigger newborns is a good thing. This points to improvements in health interventions for both mothers and offspring but the larger a baby is, the more difficult childbirth becomes.

In the past, if a baby was too big to pass naturally through the birth canal, it would spell a disaster for both mother and child. Mitteroecker explained that is is selection from an evolutionary perspective.

"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," he said.

Based on their estimates, the researchers put global cases of fetopelvic disproportion as occurring in about 3 percent, or 30, for every 1,000 births. However, this rate has gone up in the last 50 or 60 years, jumping to 3.3 to 3.6 percent, or as many as 36, in every 1,000 births.

Mitteroecker and colleagues are concerned about the future. With the original rate already up 10 to 20 percent due to C-sections' evolutionary effect, it leaves a pressing question of what's the future rate going to be like.

The researchers clarified that they are not criticizing medical intervention but do want to acknowledge that C-sections have an evolutionary effect. And while more babies may be born via C-section in the future, the researchers don't think that there will come a day when every baby will have to be delivered with the procedure.

For the study, Mitteroecker was joined by Simon Huttegger, Barbara Fischer and Mihaela Pavlicev.

Cesarean Births Statistics

According to the National Center for Health Statistics, there were 3.978 million births in the United States in 2015, a figure one percent less from last year's. The rate for cesarean deliveries actually dropped to 32 percent for 2015, with overall and low-risk cases of C-sections declining for the third year in the row.

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