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...a 2007 study on the evolutionary reasons pregnant women don't tip over.
Scientists(as well as far more casual observers of the human condition) have long noticed that pregnant women lean back to avoid toppling over. But that can put extra pressure on their spines, leading anthropologists to theorize that women's lower vertebrae evolved to reduce such pressure during pregnancy.
Shapiro and her fellow researchers found out just how. By studying 19 pregnant women, they discovered that a woman's lumbar, or lower back, curve extends across three vertebrae. In men, it extends across two. The joints between the vertebrae also are larger in females and angled differently from those of males to better support the extra weight.