New Theory on Why Pregnancy Last Nine Months

A new paper says the main reason babies are born helpless around nine months is because mom can't handle the metabolic strain.

(Image source: Wikimedia Commons ) BY STEVEN SPARKMAN   ANCHOR MEGAN MURPHY Pregnancy lasts 40 weeks, and babies are born helpless. These are facts most people take for granted, but have you ever wondered why? You might have if you’re a very pregnant woman, a new parent or — like the authors of a new study — anthropologists. Here’s WNWO . “Researchers at the University of Rhode Island, Harvard and the University of California-Berkeley say that it’s really about how much energy mom can spare for the developing fetus.” The theory that’s ruled for decades is called the obstetrical dilemma. Basically, human pelvises have to be a certain shape to allow us to walk upright, but that limits what can pass through. LiveScience explains . “If babies were born with bigger brains, the theory goes, they'd get stuck in the birth canal. Instead, they stop gestating before they grow too large, resulting in completely dependent newborns.” So, humans are pushed out into the world a little underdone. Babies’ brains are only 30 percent developed, compared to baby chimps’ 40 percent. But if mom’s hips were wider, she’d have trouble walking. Except the researchers didn’t find any evidence to support that theory. Scientific American reports that to fit infants with “a chimplike stage of brain development,” the birth canal would only have to be three centimeters wider on average. Many women have hips that wide and don’t suffer any walking penalties. The researchers say there must be another factor limiting pregnancy length, and NPR reports — they point the finger at energy. “In fact, pregnant women's metabolism runs at twice the normal level by about the sixth month. By nine months, as the fetus's energy needs increase, the rate is pushing close to 2.1 times normal. And that's pretty much the limit.” The researchers found that throughout the animal kingdom, it’s mom’s metabolism, not her hips, that determine when she has to give birth. But the study’s lead author writes on her blog it may take awhile to replace the old theory. “As we were putting our story together, I joked to a brand new mother, ‘You know, there's no obstetrical dilemma.’ And got a sharp-tongued, Oh yes there is, honey. I just went through labor. Hell yes there is!” The study will be published in the Proceedings of the National Academy of Sciences .