Tummy sleeping A baby's risk of SIDS (Sudden Infant Deaths) has been found to be 1.7 to 12.9 percent higher (depending on the study) if he sleeps on his tummy instead of his back. According to Betty McEntire, the executive director of the American SIDS Institute, when a baby sleeps tummy-down he's more likely to overheat, have pauses in breathing, and re-breathe the air he has just exhaled, which can raise his level of carbon dioxide.
In addition, several studies have shown that if a baby is used to sleeping on his back and is placed to sleep on his tummy, his risk of SIDS rises markedly. (This may account for the unexpectedly high rate of SIDS in daycare settings.)