Although this has biology in it, I still think it qualifies more as a physics question. If you take person and hang them upside-down long enough, blood will pool in their head and they will eventually die because of the pressure on their brain. I know that in the uterus, an unborn baby is upside down for many weeks. What is it about the water (buoyant force?) that keeps blood from pooling in a baby's head? Gravity is still acting downwards.