Why pregnant women don’t tip over

Women have some reinforcements in their backs that men lack. This is to compensate for the forward shift of the trunk’s centre of mass (COM) above the hips during pregnancy.
Isn’t nature beautiful?

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.

From Scientific American

bipedalism poses a unique challenge to pregnant females because the changing body shape and the extra mass associated with pregnancy shift the trunk’s COM anterior to the hips. Here we show that human females have evolved a derived curvature and reinforcement of the lumbar vertebrae to compensate for this bipedal obstetric load.

From Nature