A pregnant woman's lower spine has a hidden flexibility which shifts her centre of gravity backwards so she does not topple forwards due to the weight of her baby, researchers have found.
An analysis of the female backbone has shown how the spinal curvature of a woman becomes more pronounced when pregnant. This ensures the weight of the developing foetus is placed directly over the pelvis and reduces pain and fatigue for the woman's back muscles.
The same researchers have shown that the male spine does not have the same features and so is inherently more rigid.
This suggests the flexibility of the female spine has come about because of the extra strains put on the upright posture during pregnancy, scientists said.
"Pregnancy presents an enormous challenge for the female body. It must change in dramatic ways to accommodate the baby, and these changes affect a woman's stability and posture," said Katherine Whitcome of Harvard University.
"Enhanced curvature and reinforcement of the lower spine are key to maintaining normal activities during pregnancy," Dr Whitcome added.
It is the first time anyone has demonstrated a difference between the sexes in the vertebrae of the human lumbar region the lower back according to the scientists, whose study is published in the journal Nature.
Dr Whitcome said the natural curvature of the lower back, which is known as lordosis, stabilises the upper body above the lower body.
This is especially important during pregnancy because of the need to shift the centre of gravity backwards to compensate for the extra weight at the front.
The study analysed the posture of 19 pregnant women. When standing, the women leaned back slightly to increase their lordosis by as much as 60 per cent by the time they were about to give birth.
Dr Whitcome said: "In females, the lordosis is subtly different than that of males, because the curvature extends across three vertebrae, while the male lordosis curves across only two vertebrae.
"Loading across three vertebrae allows an expectant mother to increase her lordosis, realigning her centre of gravity above her hips and offsetting the destabilising weight of the baby."
In addition to being able to curve the spine more during pregnancy, the joints in the vertebrae of women are relatively larger than in men and flare out further down the spine, which improves the strength of the lower spine allowing women to lean back further. Lisa Shapiro, an anthropologist at the University of Texas at Austin, said female chimpanzees do not show this adaptation while the spine of a female Australopithecus a two-million-year-old primate which walked on its feet and had human characteristics does show the change. "Natural selection favoured this adaptation because it reduces extra stress on a pregnant female's spine," Dr Shapiro said.
"Without the adaptation, pregnancy would have placed a heavier burden on back muscles, causing considerable pain and fatigue and possibly limiting foraging capacity and the ability to escape from predators," she said.
"Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body. Yet our research shows their spines have evolved to make pregnancy safer and less painful than it might have been if these adaptations had not occurred."