Scientists believe they may have explained why babies born to smokers are at increased risk of cot death.

They have discovered that exposure to tobacco smoke in the womb appears to slow down a baby's arousal response.

Previous research has indicated that cigarette smoking by the mother could be implicated in up to 30% of cases of cot death.

Smoking and cot death

Smoking during pregnancy increases cot death risk by up to 15 times The risk of cot death increases according to the number of cigarettes women smoke per day while pregnant Babies who share a bed with parents who smoke are more likely to die of cot death

Researchers from the Royal Children's Hospital in Herston, Queensland, Australia, monitored the sleep and arousal patterns of 20 babies between the ages of 8 and 12 weeks, which is when cot deaths peak.

Half the babies' mothers smoked during pregnancy; the other half were non-smokers.

All of the babies had been born at term and none was sick or had been receiving any treatment.

Sound test

Each baby was given a specialised test to measure the way their bodies reacted to a sudden sound while asleep.

This paper adds weight to the evidence that smoking must be avoided for the well-being of small infants Foundation for the Study of Infant Deaths

Faced with a sudden shock to the system, automatic physical processes are triggered that are designed to keep a baby's airways open, and protect against a life threatening shortage of oxygen.

The test was designed to measure how fast these physical safeguards kicked in.

The research revealed no differences in overall sleep patterns among the babies.

Neither was there any difference in arousal times when the babies were tested during the Rapid Eye Movement, or dream phase, of sleep.

But five of the babies whose mothers smoked during the pregnancy did not open their eyes or begin to move when the test was applied during non-REM, or deep, sleep.

The researchers believe their findings may partly explain the link between smoking and cot death.

Lack of oxygen

It is thought that tobacco smoke may act to reduce the supply of oxygen to the brain, leading to stunted mental development, and hence slower reaction times.

Alternatively, nicotine may disrupt the genetic control of the body's mechanisms for dealing with stress.

Researcher Professor Anne Chang told BBC News Online: "It would be impossible to prove the actual mechanism of death in all sudden infant death syndrome [cot death] victims and it is likely that SIDS is the end point of variety of abnormalities and thus a variety of mechanisms probably exist.

"The link between altered arousal and SIDS is however strong."

In a statement, The Foundation for the Study of Infant Deaths (FSID) said: "This paper adds weight to the evidence that smoking must be avoided for the well-being of small infants.

"FSID advises parents to cut smoking in pregnancy, keep their babies smoke free and avoid taking their baby into smoky places".

Sinister effect

Clive Bates, director of the anti-smoking charity ASH said: "This suggests smoking during pregnancy has subtle and sinister impacts deep within the mechanisms that control consciousness of the unborn baby.

"The best any mother can do for herself, her baby and the family is quit smoking, and help is now available throughout the NHS to make it happen."

There is evidence linking smoking in pregnancy with an increased risk of spontaneous abortion, low birth weight, sudden death and disease in babies.

Ministers are committed to cutting the percentage of women who smoke during pregnancy from 23% to 15% by the year 2010.

The research is published in the journal Archives of Disease in Childhood.