Babies sleep better on their tummies if unwrapped because they cannot wake themselves up by their startle reflex (as on their fronts their arms can't move in the direction of the reflex). If you wrap a baby firmly this will have the same effect and they will not wake themselves up with their arms. Was your baby wrapped firmly in the hospital? Did they sleep okay then? Some babies will still prefer their tummies, but I think most will sleep well on their backs if wrapped.