Normal people don’t really think about stuff like this, but babies should really be born ready to walk, or at least crawl around. What other species bears helpless little creatures that can’t even run away from an angry snail? All manner of vegetation, sure; but mammals — not so much. (Incidentally, this observation led ethicists from Oxford to deduce that there’s no moral difference between abortion and infanticide: a fetus and a newborn are the same, except for which side of the birth canal they’re on; both are helpless, unaware, and fully dependent on others for their existence. But that’s a whole other issue.) Up until now, the going theory as to why human gestation isn’t 18 months, like it should be, has been that the mom couldn’t give birth to such a large baby — at least not without her pelvis being left somewhat crippled.
That didn’t sound quite right to some academics, so they did what academics do best: research. Long story short, they figured out that women give birth to babies that large all the time — to deliver a baby the size of a 9-month old, they would only need an inch bigger than average dilation of the birth canal. So the pelvis thing couldn’t be the problem.
Enter the alternate theory: pregnant women burn a lot more calories, because making a person apparently takes some elbow grease. In fact, six months into the process, their metabolism burns twice as high as normal and stays there until birth. So the new theory is that metabolism turbo-charged to that level can only be sustained for so long, not to mention that it would have to go even higher for an 18-month fetus. Mom’s body would probably call it quits long before the baby’s ready to crawl upon delivery. But once science figures out how to give men uteruses (uteri?)… watch out, diaper industry!