NEW YORK (Reuters Health) - Smoking during pregnancy is never a good idea, but new research suggests that genes may amplify the harmful effects of cigarette smoking in some women.

In a study of more than 700 mothers, smokers were more likely to have a low birth weight baby, but the odds were greatest among women with certain genetic make-ups. Babies born to women who smoked during pregnancy or during the 3 months beforehand weighed an average of 280 grams less than babies of nonsmoking women, the researchers report in the January 9th issue of The Journal of the American Medical Association (news - web sites). Smokers also gave birth almost a week sooner, on average, than nonsmokers. It was not a surprise that babies born to mothers who smoked were more likely to have a low birth weight, according to lead researcher Dr. Xiaobin Wang of Boston University in Massachusetts. ``More importantly, our study demonstrated that maternal cigarette smoking was even more harmful in a subset of women with certain metabolic genotypes,'' Wang told Reuters Health. She and her colleagues found that variations in two genes, CYP1A1 and GSTT1, which are involved in metabolizing byproducts of cigarette smoke, affected the risk of low birth weight. The risk of having a low birth weight baby was increased 1.3 times in smokers with the AA version of the CYP1A1 gene, but the risk was increased 3.2 times in women with another version of the gene, Aa/aa. The investigators observed a similar pattern when they grouped women according to what version of the GSTT1 gene they carried. The risk of having a low birth weight baby was increased 1.7 times in smokers with one version of the gene and 3.5 times in those with a different version of the gene. ``This is one of the first studies to show specifically that both maternal genes and environmental exposure can interact to affect infant birth weight,'' Wang told Reuters Health. This suggests that some genes involved with metabolism may interact with cigarette smoking, she explained. Women with these gene variations may be ``particularly susceptible'' to the harmful effects of smoking, Wang noted. The findings raise the question of whether pregnant women should be screened to see whether they carry gene variations that make cigarette smoking more dangerous. But Wang cautioned that much more research is needed before the current results will have any practical implications. The phenomenon of low birth weight is very complex, Wang noted. ``Many environmental and genetic factors may be involved,'' she said. ``Our hope is that this work will open up new areas of research that will lead to better understanding of the causes and biological mechanisms of low birth weight,'' Wang said. This will ultimately lead to reductions in low birth weight and infant mortality, she added. In an editorial accompanying the study, Drs. George P. Vogler and Lynn T. Kozlowski, of Pennsylvania State University in University Park, point out that other genetic factors besides the ones used in the study affect the metabolism of cigarette smoke. Still, although it is not yet practical to genetically classify all pregnant women, the CYP1A1 and GSTT1 genes ``might be considered in the future for inclusion in risk screening strategies as genetic risk assessment becomes more routine and cost-effective,'' the editorialists conclude.

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