'A potential explanation for our unexpected results is that when a pregnant woman smokes, the tobacco affects her fetus's DNA. We 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.'

The harmful effects of tobacco products can be passed through the generations, the study suggests, even if the damage is not visibly apparent in the second generation.

"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," said Dr. Gilliland.

Risk of Asthma Can Double

Keck School of Medicine researchers analyzed data from telephone interviews with parents or guardians of 908 children. A total of 338 of the children had asthma within their first five years of life, and 570 served as a control group.

The study showed that children with mothers who smoked while pregnant were 1.5 times more likely to develop asthma early in life, and those children with grandmothers who smoked while pregnant, were 2.1 times more likely to develop asthma.

Further analysis revealed that even if a child's mother did not smoke while she was pregnant, but the child's grandmother did, the child was 1.8 times more likely to develop asthma. If both the mother and grandmother smoked while pregnant, a child was 2.6 times more likely to develop asthma.

Tobacco May Affect Fetus's DNA

"A potential explanation for our unexpected results is that when a pregnant woman smokes, the tobacco affects her fetus's DNA," said Dr. Gilliland. "We 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."

When a pregnant woman smokes, biological damage is done to her fetus, the researchers conjecture, and the chemicals from the tobacco can affect the child in two ways.

First, if the child is female, her eggs can be affected, which puts her future children at risk.

Second, damage may be done to the fetus's mitochondria, which is then transmitted through the maternal line, as well.

Either effect can put a woman's children and grandchildren at an increased risk of asthma by decreasing their immune function.

Underscores Importance of Smoking Cessation

"These findings indicate that there is much more we need to know about the harmful effects of in utero exposure to tobacco products," said Paul A. Kvale, MD, FCCP, President of the American College of Chest Physicians.

"They also demonstrate how important smoking cessation is for both the person smoking and their family members," Dr. Kvale emphasized.