Sharing a bed with their parents increased the risk of sudden infant death syndrome (SIDS) in babies under three months old by at least a factor of five, even without any other risk factors, the largest ever analysis of individual cases suggests.

Sudden infant death syndrome (SIDS), also known as crib death, refers to the sudden and unexpected death of an apparently healthy infant under one year of age, although it tends to peak between seven and 10 weeks of age. It is the leading cause of infant deaths in developed countries, including Canada. According to the Canadian Foundation for the Study of Infant Deaths, about 1 in 2,000 babies or about three babies per week, die of SIDS in Canada. The cause of SIDS is unknown.

Infants who share a bed with their parents have long been known to be at higher risk of SIDS. Meanwhile, babies who are breast-fed and whose parents do not smoke are considered at lowest risk for SIDS.

An international team of researchers led by Robert Carpenter, a medical statistician at the London School of Hygiene and Tropical Medicine in the U.K., wanted to find out whether bed sharing is risky even for infants in that low-risk group, and if so, how risky. 

Their study, published Monday in the journal BMJ Open found that bed sharing, sometimes called "co-sleeping," was indeed "a significant risk factor for the first 15 weeks of life in the absence of…all other risk factors." It suggested that 81 per cent of SIDS deaths for infants within this low-risk group and 88 per cent of SIDS death overall could be prevented by not bed-sharing.

"One has to ask whether it is worth taking the risk, however small, of losing a baby, when it can be so easily avoided," the researchers wrote.

The risk of SIDS was further "greatly increased" when sharing a bed if either parent was a smoker or the mother had consumed alcohol or used illegal drugs, including marijuana, at any time since the child was born, the study reported.

The Public Health Agency of Canada recommends that infants be placed to sleep on their backs in a crib, cradle or bassinet and notes that sharing an adult bed, sofa or armchair with an adult or another child greatly increases the risk of SIDS, especially in infants under four months.

However, some countries, such as the U.K. and the Netherlands, only recommend that parents avoid sharing a bed with their infant if they have risk factors for SIDS, and the study noted that there has been a "marked increase" in bed sharing in the U.K. over the past 10 years.

Current messages 'not effective'

"The current messages saying that bed sharing is dangerous only if you or your partner are smokers, have been drinking alcohol or taking drugs that make you drowsy, are very tired or the baby is premature or of low-birth weight, are not effective because many of the bed sharing deaths involve these factors," the paper said. "Our findings suggest that professionals and the literature should take a more definite stand against bed sharing, especially for babies under 3 months."

The study combined data from five published datasets from the U.K., Europe and Australasia, including 1,472 SIDS cases and 4,679 other infants. The combined data showed that 22.2 per cent of the infants who died of SIDS had been sleeping with one or both parents at the time of their death, while only nine per cent of infants in the control group had slept with their parents the night before their parents were asked about their sleep habits.

The study also calculated the amount of risk from a number of known risk factors and found that:

  • Bottle feeding increases the risk of SIDS 1.5 times compared to breast feeding when taken into consideration with other risk factors.
  • Placing a baby on its side in a crib, cradle or bassinet posed a "significant" risk (roughly double) and placing a baby on its tummy poses a "substantial" risk (12 times higher) compared to placing the baby on its back.
  • Smoking and sharing a bed with parents is a very high-risk combination – infants two weeks old who shared a bed with parents who were both smokers had a 65 times higher risk of SIDS compared to infants who slept in their own bed in the same room with non-smoking parents.
  • The mother's use of alcohol and illegal drugs, including marijuana, increases the baby's risk of SIDS five and 11 fold, respectively, even when the baby sleeps in its own bed. However, a baby at two weeks of age who shared a bed with a mother who had two or more drinks of alcohol had a 90-fold increase in risk for SIDS.