According to anthropologists, the human adaptation is unique among primates and may have arisen shortly after early humans started walking upright.
"Bipedalism challenges stable postures, because the abdomen expands in front of the body as the baby grows," said Katherine Whitcome, an anthropologist at Harvard University.
"This changes the mother's center of mass, which is a critical point in any three-dimensional body on which gravity acts."
As this center of mass shifts forward, pregnant women have to lean back and change their gait to stay steady.
Wedge-shaped vertebrae in the lower back might be the key evolutionary adaptation that helps human females maintain a stable posture over the course of pregnancy. This realigns the center of mass over the hips, knees, and ankles to correct the imbalance—but it creates another problem.
"It generates loading on parts of the vertebral column that are not normally under such stress," Whitcome said.
To find out how pregnant women keep their balance without damaging their spines, Whitcome and her colleagues studied 19 pregnant females between the ages of 20 and 40.
The team found that the key appears to be joints in the bony vertebrae that wrap protectively around the spinal cord.
These joints become heavily loaded whenever people lean back.
But the size of the joints relative to the vertebrae in the lower back is much larger in women than in men. This suggests that the joints' larger surface area is an adaptation to bear more load.
And the shape of the vertebrae in women tapers off toward the back, creating a wedge shape that further facilitates arching, Whitcome said.
Women also have three such vertebrae, while men have just two.
"These wedge-shaped vertebrae, when stacked together, form a natural curve and help reduce the shearing stress generated during pregnancy," said Whitcome, whose findings appear today in the journal Nature.
Whitcome and her colleagues suggest that the special vertebrae are unique evolutionary adaptations that helped the first ancestors of human women as they started walking upright.
For example, the researchers have found the unusual spinal characteristics in lumbar vertebral columns from Australopithecus africanus fossils dating back nearly two million years (related feature: atlas of human evolution).
"The female characteristics, which are explained by the biomechanics of fetal load, are present in the fossil record, suggesting that these adaptations evolved very early in humans," Whitcome noted.
Karen Rosenberg is an anthropologist at the University of Delaware who was not involved in the new study.
She said that the feature would have been naturally selected in humans at about the same time that bipedalism evolved, nearly five million years ago.
And John Fleagle, an anthropologist at Stony Brook University, commented that "there are lots of neat things about this paper."
"It documents some striking features of the lumbar spine of female humans that seem rather clearly related to the demands of pregnancy."
Scientists had previously known about male-female differences in the shape of the pelvis related to birthing, Fleagle added. But spinal differences between males and females had not been appreciated until now.
"Like so many discoveries," he added, "this is one that causes you to slap your forehead and exclaim, Of course! Why hasn't anyone thought of this before?"
for National Geographic News
December 12, 2007