Is it  because humans walk upright?

The prevailing explanation for why pregnancy doesn’t last that long boils down to something called the “obstetrical dilemma.” Humans walk upright. And the size and shape of our pelvises are constrained by our bipedal way of getting around in the world. If they got much bigger, mothers wouldn’t walk as well. So babies’ brains could only get so big and still fit through the birth canal, the conventional wisdom holds.

Or is it for other reasons:

“Mothers gestate a baby as long as they can metabolically,” Holly Dunsworth, an assistant professor of anthropology at University of Rhode Island, tells Shots.

She and her colleagues concluded that a human baby born at a chimp’s level of development would require the average human birth canal to be about 3 centimeters bigger, an increase of a little more than an inch in diameter.

That’s feasible, the researchers say. “We show that’s within the range of variation now,” Dunsworth says. “Those people with wider birth canals aren’t walking any worse.”

So what is the limiting factor? Apparently, it’s how much energy Mom can divert from her own metabolism to the growth and maintenance of a fetus, the researchers say. We humans are able to crank up our metabolism to about twice its normal level and sustain that turbo mode for quite a while.

From NPR.