Have you ever wondered why the human gestation period is nine months long? A recent study bucked years of traditional wisdom and revealed a surprising answer.

Scientists have long theorized that a woman’s length of pregnancy is dictated by pelvis size — in other words, you can’t let a fetus grow too much or it simply wouldn’t be able to exit a mother’s body during natural childbirth.

Au contraire, says new research. Apparently those nine months are related to a pregnant woman’s metabolism instead. By her sixth month of gestation, she’ll expend twice her usual energy output just to keep her basic metabolic processes going, and the bigger the baby gets, the more of a toll it takes on her.

So by the ninth month? Her body has had enough and expels the child. It could be as simple as that.

But maybe those pelvic-size limitations are good anyway. Our brains are less than 30 percent of their adult size at birth, which keeps newborns’ heads small and eases delivery — but if we allowed babies’ brains to get just 10 percent larger during gestation, pregnancy would last 18 to 21 months instead of nine.

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