Bob Holmes of New Scientist explains why pregnant women don’t fall over:
Women do not tip over during pregnancy because their spines are built differently from men’s – and have been ever since our ancestors began walking upright. The difference allows a pregnant woman to lean backward to counterbalance the weight of her developing fetus.
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They found that the women corrected for their growing imbalance by bending backward in the lower part of their spine. Detailed anatomical measurements showed that the vertebrae of the lower back are more wedge-shaped in women than in men, which makes this bending easier.The interlocking bony projections that align each vertebra with its neighbours are also relatively larger in women, Whitcome found. This provides added support to prevent the vertebrae from slipping sideways, as they otherwise might because of the sharper bend in the spine.
Chimpanzees, which are mostly quadrupedal, do not show these vertebral adaptations.
However, when the researchers looked at the two known fossil spines of Australopithecus africanus, an ancestral human that lived 2 to 3 million years ago, they saw that one — believed to be a female — did have the wedge-shaped vertebrae and larger interlocking projections seen in modern women, while the other — believed to be a male — did not.