Why Pregnant Women Don't Tip Over

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But the size of the joints relative to the vertebrae in the lower back is much larger in women than in men. This suggests that the joints' larger surface area is an adaptation to bear more load.

And the shape of the vertebrae in women tapers off toward the back, creating a wedge shape that further facilitates arching, Whitcome said.

Women also have three such vertebrae, while men have just two.

"These wedge-shaped vertebrae, when stacked together, form a natural curve and help reduce the shearing stress generated during pregnancy," said Whitcome, whose findings appear today in the journal Nature.

Evolutionary Adaptation

Whitcome and her colleagues suggest that the special vertebrae are unique evolutionary adaptations that helped the first ancestors of human women as they started walking upright.

For example, the researchers have found the unusual spinal characteristics in lumbar vertebral columns from Australopithecus africanus fossils dating back nearly two million years (related feature: atlas of human evolution).

"The female characteristics, which are explained by the biomechanics of fetal load, are present in the fossil record, suggesting that these adaptations evolved very early in humans," Whitcome noted.

Karen Rosenberg is an anthropologist at the University of Delaware who was not involved in the new study.

She said that the feature would have been naturally selected in humans at about the same time that bipedalism evolved, nearly five million years ago.

And John Fleagle, an anthropologist at Stony Brook University, commented that "there are lots of neat things about this paper."

"It documents some striking features of the lumbar spine of female humans that seem rather clearly related to the demands of pregnancy."

Scientists had previously known about male-female differences in the shape of the pelvis related to birthing, Fleagle added. But spinal differences between males and females had not been appreciated until now.

"Like so many discoveries," he added, "this is one that causes you to slap your forehead and exclaim, Of course! Why hasn't anyone thought of this before?"

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