Image showing a pregnant australopithecine in bipedal posture with visible fetal load and maternal vertebrae. (Image: John Gurche)

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.

One problem with bipedalism is that the growing fetus sticks out to the front, shifting the mother’s centre of mass forward of her hip joint, creating a tendency to tip over.

Pregnant quadrupeds do not have this problem, because the mother’s centre of mass remains between her front and hind limbs throughout pregnancy.


“Were a pregnant chimpanzee to stand upright, she would experience a similar problem,” says Katherine Whitcome, a biological anthropologist at Harvard University in Cambridge, Massachusetts, US.

Whitcome and her colleagues studied 19 women as they progressed through pregnancy.

Bipedal adaptations

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.

Later benefit

This suggests that even in Australopithecus, the stresses of pregnancy had caused evolution to favour these spinal changes.

“I would imagine that Australopithecus women were uncomfortable during pregnancy in the same way that modern women often are,” says Karen Rosenberg, a palaeoanthropologist at the University of Delaware, US.

The same spinal adaptations that served Australopithecus females – and still serve modern women – during pregnancy, most likely proved just as useful after birth, Rosenberg adds. After all, a woman carrying a baby in her arms may be even more front-heavy than one whose baby is still within.

Journal reference: Nature (DOI: 10.1038/nature06342)