LONDON: A new study has identified that smoking during pregnancy is the key factor behind the Sudden Infant Death Syndrome (SIDS), also known as cot deaths.

A study by the Bristol University's Institute of Child Life and Health says that nine out of ten mothers whose babies died of SIDS had smoked during their pregnancy.

It also reports that women who smoke during pregnancy are four times more likely than non-smokers to see their child fall prey to cot death.

The study, co-authored by professor of the infant health and developmental physiology Peter Fleming and senior research fellow Dr Peter Blair, is based on analysis of the evidence of 21 international studies on smoking and cot death.

"What we have been trying is to look at the whole impact of exposure, both before and after birth, to smoking and its adverse effects," says Prof Fleming.

Prof Fleming said one major concern recently was that "from the ban on smoking in public places there would be a potential increase of people smoking at home".

He added that the report, entitled "Sudden Infant Death Syndrome and Parental Smoking", calls for an "emphasis on the adverse effects of tobacco smoke exposure to infants and amongst pregnant women".

"If smoking is a cause of SIDS, as the evidence suggests it is, we think that if all parents stopped smoking tomorrow more than 60 per cent of SIDS deaths would be prevented," Dr Blair said.

The study, published in the medical journal Early Human Development this week, reports that anti-smoking messages have had some effect, with national statistics suggesting that smoking among pregnant mothers has fallen from 30 per cent to 20 per cent in the past 15 years.

But, the researchers also assert that the proportion of babies, who died from SIDS and were born to mothers who smoked during pregnancy, has risen from 57 per cent to 86 per cent.

This rise is put down to the success of the Back to Sleep campaign, launched in 1991, which said parents should lay their babies on their backs to sleep.

The campaign led to a reduction in SIDS babies and has virtually eliminated laying babies face down as a cause of cot deaths, thus leaving smoking as the chief cause.

"The risk of an unexpected infant death is greatly increased by both prenatal and postnatal exposure to tobacco smoke. We should aim to achieve a 'smoke-free zone' around pregnant women and infants," Dr Blair said.

"Reduction of prenatal exposure to tobacco smoke, by reducing smoking in pregnancy, and of postnatal exposure to tobacco, by not allowing smoking in the home, will substantially reduce the risk of SIDS," he added.