Why pregnant women don't fall over 18:00 12 December 2007 NewScientist.com news service Bob Holmes Image showing a pregnant australopithecine in bipedal posture with visible fetal load and maternal vertebrae. (Image: John Gurche) Women do not tip over during pregnancy because their spines are built differently from men's – and have been ever since our ancestors began walking upright. The difference allows a pregnant woman to lean backward to counterbalance the weight of her developing fetus. One problem with bipedalism is that the growing fetus sticks out to the front, shifting the mother's centre of mass forward of her hip...