Why pregnant women don't tip over

There seems to be more keeping pregnant women upright than fear of toppling over and squishing their unborn child. Researchers from Harvard University and the University of Texas at Austin examined 19 pregnant women and discovered a number of reinforcements in their backs that men lack, including a lumbar (lower back) curve that spans three instead of two vertebrae and spinal joints that are 14 percent larger and positioned differently. These enhancements allow expectant mothers to lean back by as much as 28 degrees more than normal to offset the added heft of a baby bump—up to 30 pounds on average, or the weight of two bowling balls—without destroying their backs, the investigators report in Nature. They also present evidence for similar differences between the sexes in Australopithecines (early relatives of humans), suggesting that women long ago evolved such scaffolding to compensate for walking upright while supporting their swelling wombs.