A new study in based on autopsies of 41 infants who died of SIDS and other causes shows that a serotonin abnormality in the brain stem that triggers breathing reflexes may be involved.

This breathing problem fits with the prevailing theory that SIDS is caused by babies breathing in too much carbon dioxide, either because they were sleeping facedown or smushed up against bedding or crib bumpers. Considering that 65% of the SIDS cases in the study were babies sleeping facedown or on their sides, it sounds like the medical community's sleep-face-up message is not getting across.

"This finding lends credence to the view that SIDS risk may greatly increase when an underlying predisposition combines with an environmental risk — such as sleeping face down — at a developmentally sensitive time in early life," said Duane Alexander, director of the National Institute of Health's Institute of Child Health and Human Development, which helped pay for the study.