According to anthropological researchers, women have a stronger and more flexible lower spine—clearly designed with childbearing in mind.

During pregnancy, the abdominal mass grows by nearly 30 percent and the center of the mass shifts forward. Normally, this would increase pressure on the spinal column, strain the muscles, and reduce stability. Women could manage the shift and avoid falling forward by leaning back, but the result would be even greater pressure on the spine and muscles.

So what is the secret?

Anthropologists have known for some time that humans have a unique forward curve in the lower spine that helps compensate for the extra pressure caused by walking upright. More recently a team of researchers investigated whether there might be a second variation in the female spine that compensates for the added strain of pregnancy.

In an effort to determine if such a variation exists, Katherine Whitcome of Harvard University and Liza Shapiro of the University of Texas studied 19 pregnant women. The results of the study are published in the December 13, 2007 issue of Nature.

They found that the lumbar curve in women extends over 3 vertebrae, as compared to 2 in men. They also found that the connecting points between the vertebrae are larger in women and shaped differently, making them more stable and less prone to shifting out of alignment or breaking when stacked one on top of the other.

Without that adaptation, said Whitcome, females would have experienced significant pain during pregnancy and might not have been able to look for food or run from predators, so the pregnancy would end, taking the genetic line with it.

As further evidence of the adaptive changes in the spine, Shapiro, who works mainly with primates, found no difference between the male and female spines of chimpanzees, suggesting that the changes in the human spine occurred in response to problems caused by upright posture.

Whitcome also examined two sets of fossilized vertebrae for signs of similar changes. She found the three-vertebra arrangement in one set but not the other. Separate evidence from both skeletons indicated that the three-vertebrae spine belonged to a female and the two-vertebrae spine to a male.

“It was very exciting to have the fossilized pieces of the puzzle fall into place,” said Whitcome.

Despite the forward curve in the lower spine, women commonly complain of back pain during pregnancy. While the forward curve supports upright posture, it is not a structurally ideal solution, since it can lead to instability and even fractured vertebrae. But according to researchers, without these adaptations, human would have even greater problems.

Although anthropologists have extensively explored the adaptive development of the female hip bones, which are known to have expanded over time to accommodate the birth of human babies with large heads, this is the first study to investigate the lower spine.

So from now on, I’m not going to complain about my large hips or sore back—I’m going to thank my ancestors, without whom I would not have been able to birth my 9 and 10 pound babies.