I think it's every parent's nightmare -- SIDS.
Researchers have found a new clue -- a serotonin imbalance, when coupled with external factors such as belly sleeping, can decrease a baby's ability to wake and move when oxygen supply is inadequate.
It's so chilling to think about it. I still check Chim and Badger every night before bed and Charlie, many many times per night. Of course that little stinker is waking up 5x a night so that's reassuring :)