DS started sleeping on his tummy at about 6 weeks. It was right after he learned to roll from back to tummy and that became the only way he would sleep (besides being held, vertical). I was scared to death to let him sleep on his belly and would always put him down on his back and would turn him all night long for weeks. I finally relaxed a little once he was able to roll the opposite way and realized nothing I did was going to change the fact that he was a tummy sleeper. Now, at 9.5 months and with mommy comfortable with tummy sleeping, he only wants to sleep on his back or side. Go figure.