Most other mammals are not bipedal though, and would need to be on their stomachs to be able to get up and flee from predators easily.
Just imagine a human newborn being placed on its front on dirt or rock, in a cave. That's NOT natural, and it's not instinctual. We have the luxury of that option because of comfy, clean mattresses - but biology doesn't know this and it is riskier to place an infant on its stomach to sleep than on its back or in a parent's arms.
Edited by Bokonon - 6/20/11 at 11:46am