A new study published in Thursday's journal Nature sheds light on the way evolution has shaped the vertebrae in human females. Anthropologists from Harvard University and the University of Texas found that their lower vertebrae evolved in ways that reduce back pressure during pregnancy.

It's well known that pregnancy causes a shift in posture, because the womb enlarges and moves the centre of gravity forward. This means that women need to lean back to maintain balance, which causes additional strain on the spine and back muscles. Also, the lumbar area curves significantly to accommodate the new posture.

"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."

Katherine Whitcome and her colleagues have studied 19 pregnant females between the ages of 20 and 40 and found that evolution had produced a stronger and more flexible lower spine for women. The lumbar curve in women extends across three vertebrae, whereas in men it affects only two.

The extra belly weight is more of a problem for humans than for other animals, because they walk on just two limbs. The evolutionary adaptation of the lumbar vertebrae in women is expected to have thus occurred just after humans started walking upright.

In fact, researchers looked back at fossil records of human ancestors, including the oldest spines that go back 2 million years to our predecessor, Australopithecus. The anthropologists found a male without the lower-back changes and a female with them.

Back pain, also known as "dorsalgia", is a common complication of pregnancy which usually occurs during the third trimester. It can be treated with mild exercise, gentle massage, heating pads, or paracetamol (brand name Tylenol), according to the recommendations received from each woman's physician.