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Scientists Discern How Pregnant Women Don't Topple

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By SETH BORENSTEIN, The Associated Press

Published: December 13, 2007

WASHINGTON - Scientists think they have figured out why pregnant women don't lose their balance and topple over despite ever-growing weight up front.

Evolution provided them with slight differences from men in their lower backs and hip joints, allowing them to adjust their center of gravity, new research shows.

This elegant engineering is seen only in female humans and our immediate ancestors who walked on two feet, but not in chimps and apes, according to a study published in today's journal Nature.

"That's a big load that's pulling you forward," said Liza Shapiro, an anthropology professor at the University of Texas and the only one of the study's three authors who has actually been pregnant. "You experience discomfort. Maybe it would be a lot worse if the design changes were not there."

Harvard anthropology researcher Katherine Whitcomb found two physical differences in male and female backs that until now had gone unnoticed: One lower lumbar vertebra is wedged-shaped in women and more square in men; and a key hip joint is 14 percent larger in women than men when body size is taken into account.

The researchers did engineering tests that show how those slight changes allow women to carry the additional and growing load without toppling over - and typically without disabling back pain.

"When you think about it, women make it look so very ... easy," Whitcomb said. "They are experiencing a pretty impressive challenge. Evolution has tinkered ... to the point where they can deal with the challenge.

"It's absolutely beautiful," she said. "A little bit of tinkering can have a profound effect."

The back changes appear to have evolved to overcome the cost of walking on two feet, said Harvard anthropology Professor Daniel Lieberman.

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