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Scientists believe they have solved the mystery of cot death, the leading killer of children in their first year of life.
A study of victims of sudden infant death syndrome (Sids) has revealed that they had an abnormality in the brain that prevented it realising their bodies did not have enough oxygen.
The effect of the abnormality is that babies may suffocate if they become smothered by bedclothes, especially if sleeping on their fronts.
The finding "takes the mystery away from Sids", said Marian Willinger, a researcher at the US National Institute of Child Health and Human Development, which financed the study.
"It should take the guilt away from any parent who has lost a baby, because they always wonder, 'What did I do wrong?' Now, they need to understand, 'My baby had a disease'."
The researchers from Boston Children's Hospital said the finding was the strongest evidence yet of a common cause for cot death, the cause of which has been a mystery to specialists.
It opens the possibility of detecting infants at risk - possibly through a scan in the womb - and treating them.
Some mothers have been jailed for murder after the unexplained deaths of their babies.
In New Zealand, about 50 babies die from cot death each year.
In Britain, 300 babies died last year, 16 per cent fewer than in 2004 and less than a third of the number 20 years ago.
The Back to Sleep campaign in the early 1990s which urged parents to put babies to sleep on their backs is credited with bringing about the dramatic fall.
Greater alertness and better ways of determining causes of death have also contributed to the decline.
The researchers studied samples from the brainstems of 31 cot death babies and compared them with samples from 10 babies who died of other causes.
They found abnormalities in the brain stems of the cot death babies affecting the way they used the brain chemical serotonin.
Serotonin is best known for its role in depression and regulating mood, but it also influences breathing, body temperature and arousal from sleep.
"This finding lends credence to the view that Sids risk may greatly increase when an underlying predisposition combines with an environmental risk - such as sleeping face down - at a sensitive time in early life," said Dr Duane Alexander, director of the National Institute of Child Health.
Hannah Kinney, the paper's senior author, said: "These findings provide evidence that sudden infant death syndrome is not a mystery but a disorder that we can investigate, and some day may be able to identify and treat."
Most babies will wake up, turn over, and start breathing faster when their carbon dioxide levels rise.
But in babies who die from Sids, defects in the serotonin system may impair these reflexes.
Such circumstances are far more likely to arise if a baby is placed face down in the cot.
Auckland paediatrician Dr Shirley Tonkin, of the Cot Death Association, said: "We've never really been able to explain why some babies don't wake up, and this research explains why to some extent."
But she said parents still needed to be careful. "You can't tell by looking at a baby which one will get into trouble and which one won't.
"You still shouldn't smoke during pregnancy or around babies, you should put babies on their backs and they should sleep in their own space in the parents' room."
She wasn't surprised at the findings, saying: "We've had inklings of this before."
She hoped the findings would help parents who had lost babies to cot death.
"This research explains to parents that what they do for one child they might not be able to do for another. If one baby was OK on its tummy, the next one might not be."
- INDEPENDENT, STAFF REPORTER
