I'll answer that: my huge ass served as a sturdy anchor. So the better question is, why don't skinny pregnant twigs like Nichole Richie tip over.
I can answer that too. Anthropologists who studied the spines of living and long-dead fossilized women found that the vertabrae evolved over time specifically to keep pregnant women balanced and capable of running away from predators (like the fetus-fetishizing paparazzi?)
The researcher studied the lower spine of 19 modern-day pregnant women and found the lower back extends across three vertabrae in women and just two in men. Moreover, the connecting points were larger and the bones themselves shaped in a way that allowed them to stack without shifting or breaking. All this gives the female lumbar greater range of motion, which allows them to lean back as the size of their bellies increases, keeping them from toppling over when they walk (but not helping them in the least when trying to put on boots at 37 weeks gestation).
But why the difference? Evolution, of course!
The women who had this genetic mutation survived to have babies who also survived who had babies who also survived ... all because they could forage and get away from predators despite an enormous mid-section.
It's imperfect, since pregnant women still often complain of back pain. But like many traits that evolved, it's good enough. Especially if you have a skilled masseuse.
