Shapiro-Mendoza, PhD, of the CDC, tells. She credits an "impressive" decline in SIDS cases to a national "back to sleep" campaign that, among other things, urges parents to put babies to sleep on their backs. The seeming contradictions in increases and declines in deaths of infants are likely due to changes in the way such tragedies are investigated and classified, researcher Neille K. "Or infants getting wedged between the mattress and the wall." Also, despite urgings from the American Academy of Pediatrics and the CDC, many parents still put babies to bed on their stomachs, which can lead to suffocation. The decline in SIDS rates has been offset by an increase in other sudden unexplained infant deaths, a new category called SUIDs.

ASSB death, a subgroup of SUIDs, is a leading cause of infant mortality, says Shapiro-Mendoza. The reason for the sharp increase in accidental suffocation and strangulation in bed, or ASSB, isn't known for sure. "It's probably due to poor bedding, blankets in cribs, sleeping with parents or siblings, or bad habits," she tells.

In the same period, rates of sudden infant death syndrome (SIDS) declined, while sudden unexpected infant deaths remained stagnant, shows the study, published in the February edition of Pediatrics.