"What women do when their pregnancy reaches about half of the expected mass ... they shift backwards," Shapiro said."If you didn't have any of those mechanisms, the only way to offset a load in front of you is to contract your back muscles. The more you have to use your muscles, the more discomfort you would have. It would be worse otherwise, and there would be more potential damage to the vertebrae."
I never knew this. It does clarify why pregnant women have such hollow backs.
Hurray for Evolution!

