DS slept happily on his back from birth and then one day at about 4.5months old he just started flipping over and sleeping on his tummy. It freaked me out at first and I spent alot of time flipping him back onto his back only to go back in and find him on his tummy again. I came to the conclusion that if he was strong enough to keep flipping himself over that he was old enough to sleep however he pleased. I checked on him every 5 minutes too for a long time too.