Scientists believe they may have found a cause of cot death which could be identified by a blood test.
It has been reported that babies who die of cot death have low levels of the hormone serotonin in their brains.
Cot death is also known as sudden infant death syndrome and while some risk factors are known, it is still not fully understood.
But now researchers at Harvard Medical School think babies with low serotonin levels, a hormone which controls sleep and breathing patterns, could be more at risk.
Low levels of the hormone could lead to babies not waking up or changing position when their breathing is impaired.
The scientists looked at brain tissue from babies who had died of cot death and also those who died of other causes.
Serotonin levels were 26% lower in the babies who had died of cot death, according to the findings published in the Journal of the American Medical Association.
Senior author Dr Hannah Kinney, of Harvard Medical School and Children's Hospital Boston, said: "Our research suggests that sleep unmasks the brain defect.
"When the infant is breathing in the facedown position, he or she may not get enough oxygen.
"An infant with a normal brainstem would turn his or her head and wake up in response. But a baby with an intrinsic abnormality is unable to respond to the stressor.
"It's no one single factor but a culmination of abnormalities that result in the death."
However, in 88 per cent of the SIDS cases they examined, there were two or more risk factors, such as exposure to cigarette smoke, the baby's sleeping position or an illness.
Dr Kinney is now hoping that a test can be developed to measure serotonin levels in babies so that those at most risk can be identified and protected.
Professor George Haycock, scientific adviser for the Foundation for the Study of Infant Deaths, told the Telegraph that there were already measures that parents can take to protect their babies.
"Parents can help to protect their babies by following the reduce the risk advice; don't smoke, breastfeed, sleep baby on the back in a separate cot in a room with the parents," he said.
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