The question of how pregnant women manage to avoid toppling over has been answered. Scientists have discovered an elegant piece of evolutionary engineering that allows the female spine to balance the extra load.

Differences in the lower backbones and hips of men and women have developed over millions of years to allow women to carrying a foetus while maintaining an upright posture and two-legged gait, said the study published in Nature. Women’s lower spines have evolved greater flexibility and support than men’s.

A foetus and placenta can together weigh almost 7kg (15.4 lb). The extra load shifts the centre of gravity to the front of the hips, greatly destabilising the upper body and forcing the woman to lean back.

Women’s lower vertebrae are wedge-shaped and their hip joints slightly bigger than men’s. Daniel Lieberman, of Harvard University, said: “Early human women lived very strenuous, active lives and were forced to cope with the discomfort of childbearing while foraging for food and escaping from predators. This evolution-helped early woman to remain more mobile during pregnancy, which would have been essential to survival.”

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