More than half of cot deaths occur when a baby is sleeping with a parent, with drinking or drug-taking by the adult a likely contributory factor, a study suggests.

An analysis of all unexpected infant deaths, from birth to two years old, in a four-year period in the South West showed that the majority occurred when a parent was with the child, often asleep on the sofa.

The number of cot deaths has dropped dramatically since the early 1990s, with the help of better health awareness programmes. But experts said that more work was needed to reach different social and cultural groups with advice to avoid dangerous sleeping arrangements.

The team of researchers at the universities of Bristol and Warwick studied a possible link between cot death and socio-economic deprivation by comparing the unexpected infant deaths with a control group at “high risk” — young, socially deprived mothers who smoked — as well as another randomly selected control group.

The parents were interviewed shortly after the death and information was collected on alcohol and drug use. Of the 80 unexpected deaths analysed, more than half (54 per cent) occurred while co-sleeping, compared with a 20 per cent co-sleeping rate among both control groups.

Much of this risk could be explained by the combination of parental alcohol or drug use before co-sleeping (31 per cent, compared with 3 per cent among the random controls) and the high proportion of co-sleeping deaths on a sofa (17 per cent compared with 1 per cent in the random controls), the authors said.

They said that some of the safety messages were reaching parents and could be contributing to the continued fall in the cot death rate. However, the majority of the co-sleeping deaths occurred in a hazardous sleeping environment. The safest place for an infant to sleep in its first six months was in a cot beside the parental bed, said the study.

Sudden infant death syndrome was recognised in 1969 as a category of natural death that carried no implication of blame for bereaved parents. Since then much has been learnt about risk factors and parents are now advised to place infants on their backs, or in the “feet to foot” position to sleep at the bottom of the cot, in a smoke-free environment. But it is not clear which risk messages have been taken on board in different social or cultural groups and little is known about the emergence of new or previously unrecognised risk factors.

Peter Fleming, a professor of infant health and development physiology at the University of Bristol and the lead author of the report, said: “People understand the implications of drinking and driving and the vast majority follow that advice.

So we want parents, if they’ve had a drink or taken drugs, not to co-sleep with their baby.” Professor Fleming said that many parents woke up in the middle of the night to feed their baby on a sofa or armchair, believing that it was safer than feeding them in bed when, in fact, the opposite was true.

“It is really important that parents should not fall asleep with their baby on a sofa as it is very, very dangerous,” he said. “It is 25 times more risky than having a baby in bed with you. After parents have fed a baby it is really important they put them back in their cot.”

Edwin Mitchell, a professor of child health research at the University of Auckland, added: “Presumably, alcohol and drugs impair the arousal of the adult co-sleeper. However, the dangers of this combination of behaviours are, for the first time, convincingly shown in this study.”  Professor Mitchell said that cot death was largely preventable. “It is important to monitor parents’ knowledge and infant care practices to inform health education and promotion,” he said.