Published online 12 December 2007 | Nature | doi:10.1038/news.2007.374

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How women bend over backwards for baby

Extra spinal support helps women maintain balance during pregnancy without injury.

A female australopithecine, like today's moms, used her spine to support baby's weight.John Gurche

The next time you see a pregnant woman teetering under the awkward weight of her growing belly, remember this: if she were a man, it would be even worse.

Researchers have found that the vertebrae that make up a woman’s spine have evolved to give her more support, probably to help her cope during pregnancy. The results hold true for modern mothers as well as those of their ancient ancestors, Australopithecus, who lived more than two million years ago. Vertebrae in men lack these features.

Without this added support, women would have to draw more on their back muscles to stay upright. Over the course of nine months, that could lead to muscle fatigue and back injury.

When human ancestors made the switch from walking on four legs to walking on two, they had to make several skeletal adjustments. Vertebrae increased in number and thickness to provide added support to the upper body. The spine took on a curved shape in the lower back, to shift the shoulders backwards and move the centre of mass above the hips.

“I would advise all of my male colleagues not to become pregnant.”

Karen Steudel

But the added bulk of pregnancy shifts that centre of mass forwards again, making a woman more likely to tip over towards the front. Pregnant women bring their centre of mass back over their hips by leaning back, deepening the curve at the base of their spine.

Back bend

Katherine Whitcome and Daniel Lieberman from Harvard University in Cambridge, together with their colleague Liza Shapiro of the University of Texas at Austin, measured the centre of mass of 19 pregnant women and found that they leaned back by as much as 28º beyond the normal curve of the spine, they report in Nature1. The researchers found this lowers the torque around the hip created by the baby's weight by roughly eight times.

Exaggerating the curve in the lower back can place more stress on the spine: vertebrae are more likely to slip against each other, leading to back pain or fractures. Whitcome and her colleagues found that a woman’s spine has several features that help to prevent that damage. In women, the curve in the lower back spans three vertebrae; in men, it encompasses just two. The added vertebra helps distribute the strain over a wider area.

In addition, specialized joints located behind the spinal cord, called zygapophyseal joints, are 14% larger relative to vertebrae size in women than in men, suggesting that the joints are well adapted to resist the higher force. The joints are also oriented at a slightly different angle in women, allowing them to better brace the vertebrae against slipping.

Aside from the more obvious biological factors, this makes women more suited to carrying a baby. "I would advise all of my male colleagues not to become pregnant," jokes Karen Steudel, a biological anthropologist at the University of Wisconsin, Madison.

Running with baby

The researchers looked for, and found, the same trends between males and females in two Australopithecus fossils.

The adaptations would have been important back then, says Lieberman, “Imagine strapping seven kilos to your belly and then leading a really active life hunting, gathering, running away from predators,” he says.

The results highlight the importance of a healthy lower back for survival, says Carol Ward, a palaeoanthropologist at the University of Missouri in Columbia. “A bad back really impairs your ability to move around and find food,” says Ward. “So there would be strong selection for individuals to have less back pain.” Given that until recently, women spent much of their adult lives pregnant, adaptations to protect the back during pregnancy would be particularly important, she says.

Whitcome is now looking at the effect of child rearing on vertebrae, and Ward notes that it would be interesting to see how these changes integrate with adaptations made for long-distance running. Some think that the need for long-distance running has provided a key selective force in human evolution (see Distance running shaped human evolution).

“But as anybody who’s been 9-months' pregnant knows, running is not the thing you want to be doing,” says Ward. 

  • References

    1. Whitcome, K. K., Shapiro, L. J. & Lieberman, D. E. Nature advance online publication, doi:10.1038/nature06342 (2007).
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