CJKPhoto via Getty Images

C-sections have been on the rise for decades, now making up more than 30 percent of all deliveries in the United States.

An intriguing new study out of Austria suggests that as C-sections have become more common, they might also be altering the course of human evolution. More babies are being born with heads that are too big for their mothers’ pelvises ... which leads, the theory goes, to more C-sections.

Why?

Before the widespread use of C-sections, larger babies and their narrow-hipped moms had a good chance of dying during childbirth ― meaning their genes weren’t passed on.

But because C-sections tend to save those babies, the genes that contribute to a possible bigger head/smaller pelvis scenario have continued to be passed on.

The researchers believe that as so-called “fetopelvic disproportion” continues to increase (meaning, more babies have a harder time passing through their mom’s pelvis), C-section rates will grow ― although they caution that at this point it’s only a mathematical theory.

“To my knowledge, this has not been shown empirically yet,” study researcher Philipp Mitteröcker, an evolutionary biologist at the University of Vienna told Vox. His team’s findings were published in the journal Proceedings of the National Academy of Sciences (PNAS) this week.

And yet some OB-GYNs don’t think the prediction is all that far-fetched.

“It seems like a very reasonable theory,” Dr. Mari Charisse Banez Trinidad, an obstetrician at the Mayo Clinic, told The Huffington Post. “Look at obesity rates in our country and worldwide. As our weights are increasing, so are our babies’ weights. It’s not a far-off proposition that if you have bigger babies, they won’t fit as easily through the pelvis. And if you have pelvic disproportion, C-section is the safest way to deliver.”

The notion that babies’ heads are frequently too big for women’s pelvises can be divisive within some circles of the birthing world. Proponents of “natural” (i.e., drug-free, vaginal birth) sometimes warn women are being sold a “myth” that their hips are too small and their baby’s head is too big. Women have been successfully birthing babies vaginally forever, they argue. But Trinidad said that the new theory might, in fact, lend credence to the idea that babies and women are actually changing.

Nonetheless, the high C-section rate in this country has been identified as a major public health concern, one that the American College of Obstetrics and Gynecologists has attempted to address, offering guidelines for the prevention of a woman’s first C-section (which often leads to more surgical births if she goes onto have more children). Though there are many reasons why women have C-sections ― most of which nothing to do with head or pelvis size ― hospitals have been criticized for having policies in which a big baby leads to an automatic C-section.

Because during vaginal deliveries, babies heads do mold, Trinidad emphasized ― up to a point.