Women Have Backbone for Pregnancy

A pregnant mother and her young son walk past a mural in Manila. i i

A pregnant mother and her young son walk past a mural in Manila. Joel Nito/AFP/Getty Images hide caption

itoggle caption Joel Nito/AFP/Getty Images
A pregnant mother and her young son walk past a mural in Manila.

A pregnant mother and her young son walk past a mural in Manila.

Joel Nito/AFP/Getty Images

There has always been some doubt whether men have the backbone to handle pregnancy. Now science has a definitive answer: They don't.

Pregnancy places unusual demands on creatures who walk upright, something Shannon Rhodes, a software programmer in Arlington, Va., is beginning to notice.

Rhodes is seven months pregnant and "thinking (about) the balance issue."

"Those icy sidewalks we had last week — those were scary for the first time," she said.

Despite a shifting center of gravity, Rhodes says pregnancy has been a lot easier than she expected.

"I keep hearing that there is no such thing as a pregnant woman who doesn't have lower back-issues, so I think I'm a fluke," she said.

Or maybe just highly evolved.

A new study shows that women — and only women — have spines that are specifically adapted to carrying the weight of a fetus out front.

Liza Shapiro is one of the study's authors and an anthropologist at the University of Texas at Austin. Shapiro says a woman's body has had to solve a physics problem.

"It's just like when you're trying to carry a large box in front of you," she said. "The weight of the box is trying to pull you forward, so in order to offset that weight, you have to ... contract your back muscles to push your spine the other way."

Ordinarily, this would put a huge strain on the muscles in your back. But when Shapiro and two other researchers studied 19 pregnant women, they discovered this isn't the case.

Shapiro says she and her colleagues realized that women are able to carry most of their baby weight on their backbone.

Early in pregnancy, she says, a woman's spine has just a hint of an "S" shape.

"Throughout the pregnancy, as the fetus gets bigger and bigger, women are actively extending their spine, moving their upper body backward and increasing the curve of the lower spine," she said.

The lower bend in the "S" shape becomes much more pronounced. This allows bone to support much of the weight that would otherwise be carried by back muscles.

Shapiro says it is all possible because a woman's lower back has vertebrae shaped like wedges. And these vertebrae act like the wedge-shaped bricks or stones that architects use to create arches.

Katherine Whitcome, an anthropologist at Harvard, is the study's lead author. She says women have more of these special vertebrae than men do.

"A female has to grow a baby," she said. "The biomechanics of this growing baby, the increasing mass, challenge a female in a way that a male is never challenged."

So in the course of evolution, women's bodies have adapted.

Expectant mothers do get backaches. But they can still do all of the things that allow them and their offspring to survive.

Whitcome says a man's spine could temporarily assume the same shape as a pregnant woman's, but not for nine months.

"It doesn't cause instant failure," she said. "But you can imagine with this long and extended loading that the vertebrae would be subject to fracture and sustain various injuries."

So there you have it: Men aren't equipped to handle pregnancy.

The new study appears in this week's issue of the journal Nature.

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