Standing could be to blame for our crooked pelvic canal

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November 15, 2021 – Giving birth can be so difficult for humans in part because the pelvis is optimized for standing, suggests a new study.

The study, published in BMC Biology, relies on computer simulations of how bones, soft tissue, and skin work together when we move. The same approach was used to identify the key elements of a perfect baseball fastball and the best marathon gait.

In the new analysis, scientists used digital images to examine how the bones and muscles around the pelvis move during daily activities and childbirth. Because the shape of the human pelvic canal does not allow for quick or easy childbirth, investigators questioned whether its dimensions were related to another human characteristic.

In monkeys, this canal is a simple oval everywhere, so labor and delivery might be easier for these species, the scientists note. But the oval in the human canal changes direction across the pelvis, causing babies to turn during childbirth so that the head and shoulders can pass.

Computer modeling with digital images suggested that the stress and pressure of standing on two legs could give the pelvic canal its twist. When the researchers used a coherently oriented oval, like that of monkeys, the pelvis tilted and destabilized the spine, making it more difficult to balance on two legs.

Our balance could be even better with an enlarged outlet in the lower part of the birth canal, the investigators found. But a wider exit could make childbirth injuries more likely, as it would require an even harder twist of the head and shoulders in the canal. The authors say the shape of the pelvic canal may represent an “evolutionary trade-off” between these competing factors.

These findings are based on computer models using digital images and do not accurately recreate the evolutionary process or the process of childbirth. But they do offer another characteristic to consider when answering the question of why human labor and childbirth is so difficult, compared to our closest living cousins.

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