Scientists have discovered how smoking during pregnancy can result in babies of low birth weight, according to a new study.

Previous research has linked smoking with negative effects on the unborn child, including low birth weight and premature birth.

Now researchers at the University of Sydney say there is a significant association between smoking during pregnancy, an effect on blood vessels and growth restriction in the unborn baby.

The effect on blood vessels is described as endothelial dysfunction; a state that can lead to reduced dilation of blood vessels, inflammation of the vascular (blood vessel) wall and an increase in the incidence of blood clots.

The researchers studied 41 pregnant women; 21 smoked ten or more cigarettes a day and 20 who were non-smokers.

Participants were monitored between week 28 and week 32 of their pregnancy and for those who smoked, the average number of cigarettes per day was 15.7.

The results, published in BJOG: An International Journal of Obstetrics and Gynaecology, revealed that the women who smoked during pregnancy had persistent endothelial dysfunction, which remained at a constant level whether one or two cigarettes had just been smoked or following a nine-hour break from smoking.

Strong links were also found between endothelial dysfunction and intrauterine growth restriction, which carries an increased risk of babies dying around the time of birth.

Birth weight was significantly reduced in the smokers, with babies of the lowest weight being born of mothers with a significantly lower endothelial function.

Researcher Ann Quinton said: "Our work provides evidence that smoking has a direct and long term deleterious effect on vascular function in pregnancy and thus provides evidence to why babies of smokers are growth restricted."

BJOG editor-in-chief Professor Philip Steer commented: "There is extensive evidence that women who smoke during pregnancy do so at great risk to themselves and to their baby.

"This research is further proof that pregnant women must stop smoking to ensure that their babies have the healthiest start.

"Women should speak with their midwife, GP or local primary care trust who will be able to provide important information and support while they break the habit."

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