This has been bugging me since my pregnancy with DD. Why is gravity different on the inside?

A baby spends a lot of time upside down in the inside, especially at the end. I'd never hold a baby upside down once they're out, so why is it different? DH says it's most likely because of the amniotic fluid and the umbilical cord and I suspect he's right, butwhy? Gravity is still gravity, so why doesn't the blood all go to their head?