Problems during pregnancy could be an indicator a baby will be at a higher risk of cot death, scientists suggest.
Babies should be put to sleep on their backs
A study in Scotland showed mothers who had placenta defects were up to three times more likely to have a baby who went on to suffer cot death.
Researchers from Cambridge University and Glasgow Public Health Board say it could be a factor in around half of sudden infant deaths.
The research is published in the New England Journal of Medicine.
HOW TO REDUCE COT DEATH RISK
Both partners should cut smoking during pregnancy
No one should smoke in the same room as a baby
Place the baby on its back to sleep
Keep baby's head uncovered
Seek medical advice promptly if your baby is unwell
In the first six months, a baby should sleep in a cot in the parents' room
Do not share a bed with your baby if you smoke, have been drinking or taking drugs or are very tired
Never sleep with a baby on a chair or an armchair
A total of 354 babies died from sudden infant death in 2003 in Britain.
It is still the most common cause of death in infants in developed countries.
In this latest research, experts looked at the records of 214,000 women who gave birth in Scotland between 1991 and 2001, of whom 114 suffered cot deaths.
All women underwent the standard tests during their pregnancies, including a check of levels of AFP, - Alpha Feto Protein - which is found in pregnant women's blood.
AFP levels are already checked during the second trimester of pregnancy as an indicator of the risk that a baby will be born with congenital defects such as spina bifida or Down's syndrome.
But higher AFP levels can also indicate placental abnormalities.
It was found that those women who had higher AFP levels had a greater risk of having a baby who died from cot death.
There were 7.5 cot deaths per 100,000 births among women with the highest AFP levels, compared to 2.7 among those with the lowest.
Gordon Smith, professor of obstetrics and gynaecology at the University of Cambridge, who led the research, told BBC News Online: "There was already a suggestion that cot death and stillbirth are related conditions, and we know that AFP levels are related to stillbirth risk."
He added: "This is significant research. But it is not significant because we will be able to offer women a diagnostic test, it is significant because it helps us understand the causes of cot death.
"It also shows conditions in pregnancy are a major determinant of the vulnerability of the baby to cot death."
But Professor Smith said the risk was still relatively low - those women in the study with the highest AFP levels had a one in 1,000 risk of cot death, compared to an average risk of one in 2,000.
He said mothers should not worry if they have higher AFP levels, but should follow the rules for reducing the risk of cot death.
"A bad environment is bad for any baby."
Dr Richard Wilson, a consultant paediatrician and a spokesman for the Foundation for the Study of Infant Deaths, said: "This is excellent research. The Foundation for the Study of Infant Deaths will continue to fund research on avoidable risk factors for SIDS. "
But he added: "It's crucial that all parents follow the proven advice to reduce the risk of cot death by sleeping babies on the back, keeping them free from cigarette smoke and not letting them get too hot."