Asking the important questions

Why don't pregnant women tip over? (BarkingUpTheWrongTree):

[...] 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. The researchers say the extra back support evolved about 2 million years ago, and likely came about as a direct result of walking upright.