The negative health effects of smoking are well documented, but new results suggest that the dangers for pregnant women may be even more far reaching than previously believed. Scientists have found that the grandchildren of women who smoked while they were pregnant may have up to double the risk of developing childhood asthma compared to grandchildren of women whose pregnancies were smoke-free.

Researchers led by Frank D. Gilliland of the University of Southern California (USC) analyzed data collected as part of the ongoing USC Children's Health Study. They interviewed the parents or guardians of 908 school-age children, 338 of whom had developed asthma by the age of five. They report in the April issue of the journal Chest that children of women who smoked while pregnant were 1.5 times as likely to develop asthma as the offspring of nonsmokers were. If both the mother and grandmother smoked during pregnancy, the risk increased to 2.6 times that of children of nonsmokers. Most surprising, even when a mother did not smoke while she was pregnant her child had nearly double the risk of developing asthma as a child from a smoke-free home if her mother had smoked during pregnancy. "This is the first study to show that if a woman smokes while she is pregnant, both her children and grandchildren may be more likely to have asthma as a result, " Gilliland says. "The findings suggest that smoking could have a long-term impact on a family's health that has never before been realized."

Further studies are needed to confirm the new transgenerational findings, the scientists say. For now, Gilliland explains, he and his colleagues "speculate that the damage that occurs affects the child's immune system and increases her susceptibility to asthma, which is then passed down to her children."