Overall, Sudden Infant Death Syndrome has dropped in Britain over the past 20 years and a national campaign has made parents more aware of risk factors, according to infant health professor Peter Fleming of the Royal Hospital for Children in Bristol in a paper published online in The Lancet medical journal.

But researchers reported a spike in the final years of the two-decade study -- from one to four-- in the numbers of babies who died while sleeping on a sofa or in an armchair with their parents.

"Although the reasons for the rise in deaths when a parents sleeps with their infant on a sofa are unclear, we strongly recommend that parents avoid this sleeping environment," Fleming said in a statement.

Tempting to Keep Baby Close but 'Highly Risky'

SIDS is defined as a sudden death of an infant, often while sleeping, that remains unexplained even after an autopsy and death scene examination.

In Britain, an average of 135 babies die every year sleeping in their parents' beds, according to the London-based Foundation for the Study of Infant Deaths.

In addition, about 30 babies die while sleeping on the sofa with their parents, something the foundation calls "highly risky."

Keeping a baby close while having a nap is a tempting proposition for new parents, said Lisa Cohen, the chief executive of the Scottish Cot Death Trust.

"Our advice would be to cuddle, feed, and settle, but always put the babies back into their own cot or crib," Cohen said.

Exposure to Smoke Linked to SIDS

Fleming's team examined 369 unexpected infant deaths in the English city of Avon over 20 years, beginning in 1984. The number of sofa deaths rose from one in the first years of the study in 1984-1988 to four from 1999-2003.

Male, premature and low-birthweight babies, those who sleep on their sides or front, and those born to "deprived families" are at a higher risk for SIDS.

Other factors that put children at risk include smoking during pregnancy and exposure to secondary smoke after birth, the researchers said.