"Exposure to cigarette smoke, either when the baby is in utero or after the baby has been born, is a very important risk factor for SIDS," Krous said.
Physical risks around the time of death included having the head covered; sleeping on an adult mattress, couch or playpen; soft bedding; bed sharing, and having cold symptoms.
Bed-sharing increased from 19 percent to almost 38 percent during the study period.
Dr. Carl Hunt, a research professor of pediatrics at the Uniformed Services University of the Health Sciences in Bethesda, Md., commented on the findings.
"The most striking thing is that SIDS mortality rates have fallen considerably over this period of time, and the prevalence of prone sleeping has decreased significantly," Hunt said.
He noted that bed-sharing increased the most with infants younger than 2 months of age. The issue of bed-sharing has been very controversial, because of "concern that [an] across-the-board ban on bed-sharing could impede or impair the chances of successful breast-feeding."
And, he added, "with women who breast-feed, it's associated with a lower incidence of SIDS. It doesn't mean that it's causal, but there is a strong association."
The study noted that even if all babies were placed to sleep on their backs from now on, SIDS would not be eliminated.
Overall, prevention efforts should shift in a new direction, the researchers concluded.
"The most important finding of our study is to not focus on a single risk factor to be avoided, but rather the avoidance of multiple risk factors simultaneously," Krous said.
"Babies should not be bed-sharing while they are on their stomach and after they have been exposed or while they're being exposed to cigarette smoke," Krous said. "They shouldn't be placed in environments that are excessively warm. They shouldn't have their heads covered by a blanket. They should not be put on a soft sleeping surfa
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