Why Pregnant Women Don't Tip Over

About a year ago, Discover Magazine assigned me to write a story covering a Harvard study about intriguing evolutionary adaptations in the female spine. A look into “why pregnant women don’t tip over,” as Nature, the journal that published the research, cleverly called it, the study found evidence that the female spine evolved differently from the male’s, and that these changes apparently help lessen the biomechanical strains of pregnancy.

Publication of the story kept getting delayed for various reasons out of my control—this is a common (and fun!) aspect of being a freelance journalist—and eventually was killed. It made me sad. It was such a great little story.

Insh’allah, my back won’t start to hurt for a while. Still, the piece has been on my mind. I’ve been meaning to follow up with the researchers to see if they’ve since investigated further.

The research was pretty widely covered, but in case you haven’t heard about it, here was my take.

Women’s lower spines may have developed extra strength and flexibility to keep them upright and reduce back pain while pregnant. By tracking 19 women over the course of their pregnancy, researchers at Harvard University and the University of Texas at Austin explored the biomechanics of human pregnancy to tease of the female spine’s evoluationary adaptations. It turns out a curvy spine helps mothers-to-be maintain balance, reduce muscle fatigue, and prevent vertebrae from shifting or breaking during pregnancy.

Hominins developed the lower back curve, or lumbar lordosis, around the time they began to walk upright. To investigate the biomechanical challenges to bipedal stability posed by pregnancy, Katherine Whitcome, then a graduate student, and UT anthropologist Liza Shapiro tracked 19 women over the course of their pregnancy. On average, the women’s center of mass shifted 3.2 centimeters in front of their hips, and out of alignment with the lower body’s supporting joints. To compensate, the women naturally leaned back, increasing the lordosis by as much as 60 percent. The research showed that this stress is partly alleviated because the lordorsis in women extends across three vertebrae. It spans only two in men. Plus, women have larger spinal joints, called zygapophyses, than men do. Chimpanzees lack the lordosis entirely. “The zygapophyses are better able to resist the spinal loading environment of pregnancy,” says Whitcome. “They are less likely to fail or fracture under constant and long duration of back extension.”

After moving to Harvard, Whitcome and primate fossil specialist Daniel Lieberman then looked to the hominid fossil record to see when the adaptation may have arisen. Two specimens from the firmly bipedal Australopithecus africanus, presumably one female and one male, showed the same dimorphic lumbar traits. How far back in hominid history the adaptation goes, or how common it is across bipedal hominid species, is unknown, because the fossilization of vertebrae is extremely rare.

Lieberman wonders whether women have given up anything up by being able to better able to stabilize their spine. “We don’t know what—if any—disadvantages there are,” he says. “This is an interesting problem, actually. One assumes there must be a trade-off or one would expect to see the pattern in men. My guess is that this configuration limits mobility a little.”

The research has inspired new areas of inquiry for the team. One is the biomechanical challenge of carrying a baby around, which doesn’t end after a woman gives birth, Whitcome notes. But while a mother carries a fetus in one place—the womb—after birth she may carry a child in any number of ways—on her hip, on the front of her body, or on her back. “We’re designing some studies to look into the biomechanics of carrying an infant. What does it mean for upper-body loading?”

Another is the fetal cranium. It’s well known that the female pelvis was under strong selective pressure to accommodate rapidly enlarging fetal skulls. Did fetal skulls in turn adapt to travel through the birth canal of an upright mother? Human faces lack a “snoutiness” common to other primates. Some hypotheses attribute the development of our flat faces to changes in diet, or a need to balance the head. But Whitcome wonders whether reproductive pressures had a hand in this change, too, as part of the whole “suite” of skeletal adaptations that accompanied bipedalism.

She suggests the spine may once again be the key to finding out. During primate labor, the infant’s snout leads the way through the birth canal. But a human baby tucks its chin and goes crown first. It can move this way because of the foramen magnum, a hole at the base of the skull through which the lower brain and spinal cord are connected. In nonhuman primates and other quadrupeds, that hole is in the back of the skull. In humans, it is found underneath the skull, which “helps us to balance our heads very nicely,” says Whitcome. “But it also limits how much we can roll our heads forward and back. It may preclude the human fetus from extending its head as it moves through the birth canal. How much would a very snouty face, as chimps have and our early ancestors had, interfere when the foramen magnum moved?”

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