Why Pregnant Women Don’t Tip Over

[mawilson 2,133]

Pregnant women do not tip over, and researchers say an evolutionary curve has

a lot to do with the reason why.

Anthropologists studying the human spine have found that women’s lower vertebrae

evolved in ways that reduce back pressure during pregnancy, when the mass of the

abdomen grows by nearly one-third and the center of mass shifts forward

considerably. That increases pressure on the spinal column, strains the muscles and

generally reduces stability.

Even without the benefit of advanced study in biomechanics, women tend to deal with

the shift — and avoid tumbling over like a bowling pin — by leaning back. But the

solution to one problem creates another, since it puts even more pressure on the spine

and muscles.

And that, report researchers from Harvard University and the University of Texas in

the current issue of the journal Nature, is where evolution enters the story.

The lower spine in humans had already developed a unique forward curve that helps

compensate for the extra pressures that arose when the primate ancestors went from

moving around on four limbs to walking upright. Researchers looked for an additional

mechanism that might have compensated for the increased strain of pregnancy as well.

What they found, said Katherine K. Whitcome, a post-doctoral fellow at Harvard and

the lead author of the paper, was evidence that evolution had produced a stronger and

more flexible lower spine for women. After studying 19 pregnant subjects, Ms. Whitcome

found that the lumbar, or lower back, curve in women extends across three vertebrae,

as opposed to just two in men. And the connecting points between vertebrae are

relatively larger in women, and shaped differently in ways that make the stack more

stable and less prone to the bones shifting out of alignment or breaking.

Since the engine of evolution runs on the passage of genes from one generation to the

next, pregnancy is a critical moment. Without that adaptation, Dr. Whitcome said,

females would have been in considerably greater pain during pregnancy and might not

have been able to forage effectively or escape predators, ending the pregnancy and

the genetic line as well.

Working at the University of Texas with Liza Shapiro, an associate professor of

anthropology who studies the primate spine, Dr. Whitcome found that the differences

between male and female spines do not show up in chimpanzees. That suggested that

the changes occurred in response to the problems caused by walking upright.

When she moved on to Harvard and started working with Daniel Lieberman, an

anthropologist with expertise in primate fossils, she was able to examine two sets of

fossilized vertebrae for the telltale signs of evolved flexibility. Of the two samples, she

found the three-vertebra arrangement in one and not in the other. As it happened,

separate evidence from those skeletons suggested strongly that the extra-curvy spine

belonged to a female and the other belonged to a male. “It was very exciting” to have

the fossilized pieces of the puzzle fall into place, Ms. Whitcome said.

As solutions go, the forward spine is only partly successful, Prof. Shaprio said, since

women still commonly complain of back trouble and pain during pregnancy. Even the

basic forward curve that promotes balance in upright walking is “not a structurally ideal

solution,” she said, since it can lead to instability and even fractured vertebrae.

And that is the difference between the way that evolution works and the way that actual

designers do their job, Ms. Whitcome said: nature tinkers. “A designer wouldn’t build

something that has a tendency to fracture your vertebrae,” she said. For natural

selection to favor one feature over another, “It doesn’t have to be an ideal solution,” she

said. “It just has to be better.” In any case, she noted, “Without these adaptations, there

would be more problems.”

If evolution provided relief for women in pregnancy, one might ask, what about the

equally awkward morphology of men with beer guts? “You’re not the first one to ask this,”

Prof. Shapiro said with a laugh, and said that their research shows that “men would not

be as well adapted to a beer gut than a woman.”

Dr. Whitcome noted that in terms of the time that the evolutionary shift occurred, some

two million years ago, “finding extra calories wasn’t likely,” so an early hominid primate

with a potbelly would have been quite a rarity.

Anthropology has extensively explored the evolution of the female hip bones, which

expanded over time to accommodate the evolutionary growth of the heads of human

babies. But Prof. Shapiro said that pregnancy and the lower spine constituted new

ground for evolutionary biology.

“Katherine was a genius for thinking of that,” she said. “And you go, ‘Hey — why didn’t we

think of that before? It seems so obvious now.’ ”

NYTimes

[Alex+R 0]

I read that, but then they gave away why it is in like the third paragraph. Boring.

[mawilson 2,133]

I read that, but then they gave away why it is in like the third paragraph. Boring.

It's still kinda funny.

"Pregnant women don't tip over."

They do if you help them!

[Alex+R 0]

I read that, but then they gave away why it is in like the third paragraph. Boring.

It's still kinda funny.

"Pregnant women don't tip over."

They do if you help them!

I can totally see you doing that. To strangers.

[mawilson 2,133]

I read that, but then they gave away why it is in like the third paragraph. Boring.

It's still kinda funny.

"Pregnant women don't tip over."

They do if you help them!

I can totally see you doing that. To strangers.

It's called "cow tipping"

[DeadPoolX 9]

I once read that, contrary to popular belief, the best position in sex while a woman is pregnant is NOT having the woman on top. When she's pregnant her center of gravity is off and she can easily topple over or off the guy before, during or after sex.

I hate to say it, but that would be humorous. Well... so long as she didn't hurt herself, but something about her just falling right over while on top cracks me up.

[mawilson 2,133]

I hate to say it, but that would be humorous. Well... so long as she didn't hurt herself, but something about her just falling right over while on top cracks me up.

[AnewKINDofFEELIN 0]

I once read that, contrary to popular belief, the best position in sex while a woman is pregnant is NOT having the woman on top. When she's pregnant her center of gravity is off and she can easily topple over or off the guy before, during or after sex.

I hate to say it, but that would be humorous. Well... so long as she didn't hurt herself, but something about her just falling right over while on top cracks me up.

For some reason I see her bouncing around like a Weeble.

[almaty 19]

I once read that, contrary to popular belief, the best position in sex while a woman is pregnant is NOT having the woman on top. When she's pregnant her center of gravity is off and she can easily topple over or off the guy before, during or after sex.

I hate to say it, but that would be humorous. Well... so long as she didn't hurt herself, but something about her just falling right over while on top cracks me up.

doggy style....you can hang on to the extra love handles

[Ban Hammer 13,966]

I read that, but then they gave away why it is in like the third paragraph. Boring.

It's still kinda funny.

"Pregnant women don't tip over."

They do if you help them!

I can totally see you doing that. To strangers.

It's called "cow tipping"

[MR2nice 0]

Very interesting.

[AussieAmerican 0]

And here I was thinking the extra weight in my booty was keeping me from tipping over! J/K

I think it hilarious they needed to do a study on this!