Standing Might Be to Blame for Our Twisted Pelvic Canal

Giving birth may be so arduous for humans in part because the pelvis is optimized for standing upright, a new study suggests.

The study, published in BMC Biology , relies on computer-based simulations of how bone, soft tissue, and skin work in concert as we move. The same approach has been used to identify key elements of a perfect fastball in baseball and the best marathon gait.

In the new analysis, scientists used digital images to examine how bones and muscles around the pelvis move during daily activities and giving birth. Because the shape of the human pelvic canal doesn’t make for a fast or easy birth, the investigators wondered if its dimensions were linked to some other human feature.

In apes, this canal is a simple oval throughout, so labor and delivery might be easier for these species, the scientists note. But the oval in the human canal changes direction through the pelvis, requiring babies to rotate during delivery so that the head and shoulders can pass through.

Computer modeling with digital images suggested that the stress and pressure of being upright on two legs might give the pelvic canal its twist. When the researchers used a consistently oriented oval, like the one that apes have, the pelvis tilted and destabilized the spine, making it harder to balance on two legs.

Our balance might be even better with a widened outlet at the lower portion of the birth canal, the investigators found. But a wider outlet could make childbirth injuries more likely because it would require an even harder twist of the head and shoulders in the canal. The authors say that the shape of the pelvic canal could represent an “evolutionary compromise” between these competing factors.

These findings rely on computer models using digital images and don’t precisely re-create either the evolutionary or birthing process. But they do offer another feature to consider when answering the question of why human labor and delivery are so difficult, compared with our closest living cousins.

Sources

BMC Biology: “The evolution of pelvic canal shape and rotational birth in humans.”

News release, University of Vienna.

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