Half of cot deaths are linked to bed-sharing but many parents are sceptical about the risk, studies have concluded.
A quarter of 500 mothers polled by the UK's Foundation for the Study of Infant Deaths (FSID) doubted that sharing a bed with a baby put them at undue risk.
But a Bristol University team's study published online in the British Medical Journal found that co-sleeping is a factor in more than 50% of cases.
Many of the deaths occurred when parent and infant slept together on a sofa.
We know that those at greatest risk of experiencing a cot death are very young mums, often single, and still in their teens, and this group is the most likely to reject safe sleep advice
Joyce Epstein of FSID
Much of this risk was linked to parents' smoking and alcohol or use of sedating drugs before bedtime.
A fifth of the cot death infants were found with a pillow and a quarter were swaddled, which may be new risk factors, say the authors.
Although cot death rates in the UK has fallen dramatically since the "Back to Sleep" campaign in the early 1990s, specific advice is still needed to help reduce these deaths even further, say the Bristol University researchers.
REDUCING THE RISK
Place your baby on the back to sleep, not on the front or side
Don't let your baby get too hot - no bonnets
Sleep your baby with their feet to the foot of the cot
Never sleep with your baby on a sofa or armchair
Sharing a bed raises the risk, especially if you or your partner smokes, drinks or takes sedating drugs
A dummy can reduce the risk, even if the dummy falls out while your baby is asleep
"Parents need to be advised never to put themselves in a situation where they might fall asleep with a young infant on a sofa."
And they should never sleep with an infant in any environment if they have consumed alcohol or drugs, the team warned.
But they said some parents might still want to share a bed with their baby, particularly if they needed to feed their infants many times in the night, and that this practice should not be demonised.
It could lead to increased numbers of tired parents nodding off on their sofa with their baby, they warned.
FSID's director, Joyce Epstein, said the findings were alarming.
"We know that those at greatest risk of experiencing a cot death are very young mums, often single, and still in their teens, and this group is the most likely to reject safe sleep advice."
The charity has launched a social networking website, www.bubbalicious.co.uk, aimed at supporting teenage parents.
However, Rose Dodds, of the National Childbirth Trust, said the study had found that there was no increased risk of bed-sharing for babies whose parents had not drunk alcohol, taken drugs, smoked, or fallen asleep on the sofa.
She said: "It is not appropriate to tell all parents not to sleep with their babies.
"It is clear from many surveys that around half of parents sleep with their babies at some point in the first six months, and around a quarter do so routinely, so we need to help them to do this in the safest way possible.
"If we demonise the parents' bed we may be in danger of the sofa being chosen. A better approach may be to warn parents of the specific circumstances that put babies at risk."
The FSID poll findings are published in Community Practitioner.
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