Until their first birthday, babies should sleep on their backs for all sleep times—for naps and at night.
We know babies who sleep on their backs are much less likely to die of SIDS than babies who sleep on their stomachs or sides. The problem with the side position is that the baby can roll more easily onto the stomach. Some parents worry that babies will choke when on their backs, but the baby's airway anatomy and the gag reflex will keep that from happening. Even babies with gastroesophageal reflux (GERD) should sleep on their backs.
Newborns should be placed skin-to-skin with their mother as soon after birth as possible, at least for the first hour. After that, or when the mother needs to sleep or cannot do skin-to-skin, babies should be placed on their backs in the bassinet. While preemies may need to be on their stomachs temporarily while in the NICU due to breathing problems, they should be placed on their backs after the problems resolve, so that they can get used to being on their backs and before going home.
Some babies will roll onto their stomachs. You should always place your baby to sleep on the back, but if your baby is comfortable rolling both ways (back to tummy, tummy to back), then you do not have to return your baby to the back. However, be sure that there are no blankets, pillows, stuffed toys, or bumper pads around your baby, so that your baby does not roll into any of those items, which could cause blockage of air flow.
If your baby falls asleep in a car seat, stroller, swing, infant carrier, or sling, you should move him or her to a firm sleep surface on his or her back as soon as possible.