An overwhelming number of pediatricians suggest placing newborns on their back to sleep. The reason for this lies in the fact that newborns cannot hold their heads up alone. It is highly possible for newborns to smother if their mouths and noses are turned face down on the bed. Studies have also shown that SIDS is less likely to strike babies that sleep on their backs. After babies grow some and gain better muscle control of their bodies the sleeping position is both less important and controlled.
Older babies are able to roll from their tummies to their backs without any help.
Regardless of how you lay your baby down he is likely to change positions in his sleep. The best position for older babies is the one they are most comfortable in. To discover your baby's favorite position watch him when he is fast asleep.