It was thought that laying a child down on their back could lead to a possible problem with aspiration if the baby were to spit up while asleep. A sound argument that made putting a baby down to sleep seem like the right idea. However, since then there has been a huge influx of research into SIDS.
What they found was surprising. Conventional wisdom of laying a baby down to sleep was NOT the best way to prevent Sudden Infant Death Syndrome. Infants who were put down on their backs to sleep were far less likely to suffer from SIDS. And as for choking on their own spit-up, even young infants could turn their heads enough during sleep to keep from choking. This led the American Academy of Pediatrics to introduce the "Back to Sleep" campaign in 1992, which urges parents to lay infants down on their backs for sleep. Since the AAP's recommendation, the rate of SIDS has dropped more than 50% in the U.S.
One concern for parents is the flat spot that develops in babies when they spend too much time on their backs. That leads some mothers to recommend putting baby on his side instead. Unfortunately, doing this runs the risk of the baby rolling to his stomach and can put undo strain on his neck. An easy way to fight that flat spot is to increase your child's tummy time during the day.
The best way to put your child down to sleep is on their back. However, if your baby is strong enough to roll onto his or her stomach it's fine to leave them in that position. But always begin sleep time by putting them on their back, even if you know they will eventually roll onto their stomach. This is especially true for infants that are2 to 4 months old; this is the time when they are at the highest risk for SIDS.