Ever Wonder Why Pregnant Women Don’t Fall Over?

lumbarlordosis copyWonder no more.

After last week’s bummer post about the massive loss of knowledge that accompanied the dawn of the Christian Era, I thought we might need a tad of a respite before we delved into the early Christian view of women and sexuality, which is also a total bummer.

So, we go back to early hominid evolution, courtesy of brand new research, and try to figure out why pregnant women don’t fall over.

Honestly, it’s a good question. As bipedalism evolved, the body developed a slight curve in the lumbar spine, to distribute center of mass over the center of the hips, thereby keeping the early upright walkers from falling over when they stood up.

Pregnancy, which, one would guess, is absolutely essential to evolution, sorta made that center of mass move, well, forward. So, why didn’t (and don’t) women simply fall forward as they got larger and larger and larger?

Enter Katherine Whitcomb, Liza Shapiro and Daniel Lieberman, from Harvard and UT Austin, and their far-reaching study: “Fetal Load and the Evolution of Lumbar Lordosis in Bipedal Hominins,” (Nature 450, 1075-1078(13 December 2007)).

In this exploration of the physics of pregnancy, the authors explain:

Pregnant mothers habitually compensate positionally to fetal load by extending the lower back. Our longitudinal study of 19 pregnant human females shows that adjustments to lumbar lordosis permit mothers to maintain a stable anteroposterior position of the COM as gestation progresses and fetal mass increases. Although full-term females extend their hips only slightly (about 5.6°±2° (mean±s.d.)), they extend their lower back by as much as 28° (18°± 10°), which realigns the COM above the hips and support base. When gravid females are experimentally constrained from exaggerating their lumbar lordosis, the COM translates by 3.2±1.1cm (P<0.0001) by the end of gestation, increasing the upper body’s torque around the hip roughly eightfold. However, when free to self-select their positional alignment, pregnant females naturally increase their lumbar lordosis, limiting anteroposterior translation of the COM within a narrow range, less than 0.3 ± 0.7 cm (P=0.5695) by term. Once obstetric load has reached a threshold of about 40% of the expected term fetal mass, this lordotic adjustment increases in relation to fetal mass (r=0.9732, P= 0.0011), thus maintaining a stable position of the COM throughout pregnancy. Extension of the lower back helps control COM position but exerts a biomechanical cost to gravid mothers in the form of shearing forces caused by the nearly 60% increase in lumbar lordosis, from a mean angle of 32° ±12° in early pregnancy to 50°±12° at term. Two measures of the deleterious effects of spinal shearing are the increased risk of forward displacement of the lumbar vertebrae and the higher incidence of lower back pain in pregnant women. Greater shearing occurs because increases in lordosis transmit relatively more spinal load along the dorsal pillar of the spine comprising the zygapophyseal joints. Typical bipedal posture directs only 16% of the total compressive load through these joints, and slight extension of the lower back redirects another 12% to the zygapophyses. During pregnancy the mean lordotic excursion of 18° shifts even greater loads onto the zygapophyses, as much as 20–40% according to published models.

Whew. In case you couldn’t follow all that, pregnant women stick their butts out farther, increasing the lumbar curve, to shift the COM back toward the pelvis. This causes the low back pain of pregnancy. ‘CAUSE YOU DIDN’T KNOW THAT.

The fun thing about this study is that they went back and looked at Australopithecus lumbar spines. They discovered that, even WAY back then, when gestations were shorter and babies were smaller, there were differences in the lumbar spinal curve between male and female skeletons. They therefore postulate that increased beveling of the lumbar vertebrae was an evolutionary adaptation in the female to compensate for the fact that they spent all of their time barefoot and pregnant.

Last week, the authors were the proud recipients of this year’s IgNobel award in Physics.

[This post was inspired by one of my new favorite bloggers, Dr. Grumpy in the House. For those of you who always wanted an inside look into the world of patient care medicine, this is the blog for you. Be prepared, though. Once you meet his cast of characters, you may never want to see a doctor or visit a hospital again. Except Dr. Grumpy. I'd go to him in a heartbeat.]

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177 Comments

Wow. That excerpt was a flashback to nursing school literature.

Very interesting stuff, but I have to admit that I'm still hanging onto the line "…WAY back then, when gestations were shorter and babies were smaller." I'm sure that did not necessarily produce healthier children back them, but MAN that sure sounds better right now.

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Um. that is interesting. I read half a line of that tiny print and my eyes crossed and a couple of synapses in my brain died of sheer fright. ("oh no! did she go back to college? what is going on???")

These are so funny. How do we know about Australopithecus gestation? Is it educated conclusions? Mainly I want to know how they got so lucky.

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Wendy and Momisodes: The Australopithecus gestation is, of course, a guess, based on the similarity of the skeleton to chimps. As the authors put it:

"Similarities in body size and life history between australopiths and chimpanzees suggest that term mass and duration of gestation for australopiths was chimpanzee-like (1,590 g at 230 days)17,18 rather than human-like (3,200 g at 290 days)."

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Wendy and Momisodes: The Australopithecus gestation is, of course, a guess, based on the similarity of the skeleton to chimps. As the authors put it:

"Similarities in body size and life history between australopiths and chimpanzees suggest that term mass and duration of gestation for australopiths was chimpanzee-like (1,590 g at 230 days) rather than human-like (3,200 g at 290 days)."

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That explains my butt, 6 pregnancies will do that.

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(Blush) I'm actually flattered that you wrote that about me! Thank you!

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Was the "Ig" in "IgNobel" a deliberate insertion? In anticipation that what was predicted was presumed (or at least hoped) to become fact? I only ask because today, well, as you know, we saw an, umm, err, possibly predictive, exhortive, Nobel prize awarded.

And to completely change the subject again and get back on topic, I assure you there was no deliberate or intentional posterior poking-out posture on my part when I was pregnant. So it's subconscious, I suppose?

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TheMother Replies:

The Ig Nobel awards are given every year by the Ig Nobel committee, honoring "achievements that first make people laugh, and then make them think. The prizes are intended to celebrate the unusual, honor the imaginative — and spur people's interest in science, medicine, and technology."

No disrespect to the Nobel prizes are intended–in fact, Nobel laureates emcee the ceremony and present the awards.

(And no disrespect is meant to this year's newest Nobelist for "promise" of peace–but it does suggest a whole new way to get a Nobel prize–why do scientists bother with all that pesky research, anyway? They should just promise to do research…

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As one whose body completely went to hell on me with my pregnancies, especially with my last kid, this explains a lot. I agree with Lawyer Mom…the act of adjustment has got to be self-conscious, because I'd never consciously stick my rear out there.

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What does it mean that I did fall over constantly while pregnant? My theory is my feet were too small to hold me up :)

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My belly felt so heavy during both twin pregnancies that I'm amazed I was able to stand up at all, most of the time. And it's funny now when I look back at old belly pics, I notice that my back was curved more than usual and my butt stuck out further. Now that I'm not pregnant anymore, my back looks fine again but my butt…well, that's a whole 'nother story. Glad to hear it's not just me though!

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