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Women's spines have evolved to be more flexible and supportive than men's to prevent them from toppling over while pregnant, scientists believe, reports Alok Jha.
An extended curve and stronger lower back allow women to shift their centre of balance easily during pregnancy, meaning they can remain stable.
Katherine Whitcome, an anthropologist at Harvard University, who led a study, said: 'Pregnancy presents an enormous challenge for the female body. The body must change in dramatic ways to accommodate the baby, and these changes affect a woman's stability and posture.'
Humans started walking on two feet early in their evolutionary history, but this position presents problems during pregnancy because a woman's centre of gravity shifts to in front of the hips, destabilising the upper body.
The upper body is normally stabilised by a curve in the lower back called the lordosis.
In the study, published in Nature, Whitcome examined 19 pregnant women between the ages of 20 and 40, and found that when standing naturally, the women leant back, increasing their lordosis by as much as 60 per cent by the end of their term. This allowed them to maintain a stable centre of gravity above the hips.
According to the researchers, a study of fossils led them to believe that the ability of human females to better carry a baby to term while standing on two feet evolved at least two million years ago.
