SHARING a bed with a baby less than eight weeks old increases the risk of cot death, a new study has shown.

But the finding brings doctors no closer to understanding why seven babies a week in the UK die of unexplained causes.

Smoking, cannabis, old mattresses, sleeping position, genetics, dummies and being too warm have all been linked with cot deaths in the past. The latest study, an examination of the evidence of different risk factors across 20 European regions, confirms some of these as important, and adds another: sleeping with the baby.

As a result, the Foundation for the Study of Infant Deaths has changed its advice to parents. Previously it had assumed that sleeping with a baby was safe and even beneficial. Only when a mother smoked or had been drinking or taking drugs was it believed to be dangerous.

But the new research, published in The Lancet, says that bedsharing can double the risk of a cot death in babies up to two weeks old, and remains a significant factor up to eight weeks. The study looked at 745 cot-death cases.

Other risk factors remain far more important, however, with sleeping position the most significant. Babies placed face-down are 13 times more likely to suffer a cot death, and six out of ten of all European cot deaths can probably be attributed to babies lying on their front or side. Campaigns to encourage mothers to lie their babies on their backs have seen cot-death rates fall to their lowest recorded level.

Another risk that is confirmed by the new study is smoking, either during pregnancy or after birth. Other studies have suggested that smoking during pregnancy may affect key receptors in the brain that control breathing, or may slow down a baby’s arousal response so that it is less likely to wake up when breathing stops for any reason.

“Smoking before, during, and after birth were highly correlated with risk,” said Robert Carpenter from the London School of Hygiene and Tropical Medicine. If a mother smoked, sharing a bed with the baby was significantly more risky, perhaps because smokers tended to be drinkers and drinking increased the risk of smothering a baby in bed. Other risk factors included covering a baby’s head with bedding, and putting it to sleep under a duvet.

Professor Carpenter said: “The risk may be substantially reduced by taking a number of simple steps, most crucially ensuring the infant is put to sleep on its back, with no bedding other than a jumpsuit or Babygro, or in a well-fitting cotton or acrylic sleeping bag of not more than two or three tog. It is safest if a baby sleeps in a cot, in the parents’ room.”

Researchers from European Concerted Action on Sids compared home environments for cot-death babies with those of 2,400 live infants in 20 centres across Western and Eastern Europe, including three regions in England — Yorkshire, Trent, and South Western.

Joyce Epstein, director of the foundation, said: “It’s very important that research of this type continues to be able to identify new ideas for safe infant care. No such monitoring is now taking place in the UK.”