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It’s science: You really are too exhausted to be pregnant any longer

Tired Pregnant Woman

New study shows human babies are born so helpless because moms reach the limits of their energy.

We’re all aware of the fact that human babies are remarkably underdeveloped and helpless in comparison with babies of other mammalian species. The reason for this has fascinated scientists for years, and the prevailing hypothesis rested on two characteristics unique to humans: upright walking and large brains.

Scientists theorized that restricted pelvic width due to the demands of upright walking, coupled with the comparatively larger size of human fetal heads, worked together to prompt earlier childbirth. Basically, the helplessness of human infants is the evolutionary trade-off for walking on two legs and being smarter than all the other species.

But that might not be the case after all. According to Smithsonian magazine, a study has found that broader hips don’t affect bipedal locomotion.

“Instead, gestation is determined by energy. Studies of mammals show that during pregnancy females reach their species’ ‘metabolic ceiling,’ the upper limit of the amount of energy they can expend. In humans, the metabolic ceiling is 2 to 2.5 times the baseline amount of energy needed during rest. Dunsworth and her colleagues say women reach that limit by their sixth month of pregnancy. Then at nine months, the energy demands of a fetus go beyond this metabolic threshold. “Extending gestation even by a month would likely require metabolic investment beyond the mother’s capacity,” the team writes.”

A study in the Proceedings of the Natural Academy of Sciences found that not only are the muscles needed to walk unaffected by pelvic width, but the pelvis would only need to expand by 3 centimeters for human babies to be born with brains as developed as chimp brains at birth. But female pelvic width already varies by as much as 4 centimeters.

What doesn’t vary is the amount of energy required to sustain two lives. Any woman who’s had a baby can tell you that it requires enormous amounts of energy to be pregnant. Even sleeping takes effort. Even sitting on the couch takes effort. By the time you’ve been pregnant for 38 weeks, simply continuing to exist is a herculean task.

Toward the end of pregnancy, we all get a little desperate. “I cannot possibly go another two weeks,” we’ll say–a statement that’s less melodrama and more factual analysis. I can’t help but think of how incredibly difficult childbirth is for human beings, and how it requires every bit of mental, emotional, and physical energy we have left. Perhaps even if we could go for another 2, 3, 4 weeks of pregnancy, we wouldn’t have the energy stores left to give birth at all.

In any case, the next time you hear a pregnant women comment on how she has no energy left, don’t assume she’s exaggerating–she’s literally not. And for you pregnant mamas out there, don’t dismiss your feelings of exhaustion or secretly suspect that you just aren’t strong enough, or disciplined enough. The act of sustaining two lives requires all the energy you have, so give yourself a break (and maybe take a nap).


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