The cause of sudden infant death syndrome (SIDS) may be a brain abnormality that affects breathing and arousal reflexes. This may explain why babies who sleep on their bellies are more likely to die of SIDS. In the facedown position, reflexes such as head turning and arousal are harder to trigger when breathing is compromised, researchers say.

The researchers performed autopsies on 31 infants who died of SIDS and 10 who died of other causes. Examining the medulla oblongata, they discovered abnormalities in nerve cells that make and use serotonin, which helps transmit messages between nerve cells in the brain.

Since 1994, when parents were urged to put infants to sleep on their backs, SIDS deaths in the United States have been cut in half. Even so, it remains the leading cause of death for infants in the first year of life.

SourcePaterson DS, et al., Multiple serotonergic brainstem abnormalities in sudden infant death syndrome, JAMA, November 1, 2006.