Women who quit smoking before pregnancy could help their future grandchildren avoid asthma.

"If a woman smokes while she is pregnant, both her children and her grandchildren may be more likely to have asthma as a result," says Frank Gilliland, MD, PhD, in a news release.

About 11 percent of pregnant women in the U.S. smoked while pregnant in 2002, says the CDC. That's 38 percent less than in 1990.

If that trend continues — and Gilliland's findings are confirmed — future generations might have a lower asthma risk as a result.

"As a public health matter, a woman who smokes should be encouraged to quit smoking, and it is especially important that it occur as soon as she is known to be pregnant," write Gilliland and colleagues in the April issue of Chest.

Smoking Dangers Start Before Birth

Baby's asthma risk isn't the only reason not to smoke while pregnant. Besides damaging the woman's body, smoking during pregnancy can also:

—Lower the amount of oxygen the baby gets

—Increase the baby's heart rate

—Increase the chances of miscarriage and stillbirth

—Increase the chances of premature birth and/or low birth weight

—Raise the baby's risk of developing lung problems

Smoking may also damage the fetus's genes, according to a study in the March 9 issue of The Journal of the American Medical Association.

Mothers' Influence

Gilliland's study included 338 children with asthma and 570 without asthma. The kids with asthma were diagnosed by their fifth birthday.

In phone interviews, the researchers noted which kids' moms and grandmothers smoked during pregnancy.

Children whose mothers smoked for any part of their pregnancy had an asthma risk 1.5 times higher than kids whose moms had never smoked. If the mothers smoked throughout pregnancy, the risk was 1.6 times that of kids with nonsmoking moms.

About one in five mothers who had ever smoked quit before getting pregnant. Their children weren't at higher risk of asthma.

Grandma's Smoking and Baby's Asthma

Skip back a generation, and smoking still mattered.

Children were at higher risk for asthma if their grandmother had smoked while pregnant. Even if a grandmother's daughter didn't follow in her footsteps, her kids were still at higher risk for asthma.

Let's say Grandma smoked during pregnancy, but Mom didn't. The grandchild's asthma risk was almost twice as high as that of kids from smoke-free families (1.8 times).

If Grandma and Mom both smoked during their pregnancies, the child's asthma risk was even higher — 2.6 times that of children from nonsmoking backgrounds.

The intensity of smoking during pregnancy or secondhand smoke exposure after birth didn't affect asthma risk. Substantial evidence shows that secondhand smoke can cause asthma flare-ups, but the link with asthma risk is less clear, write the researchers.

Stop Smoking: the Sooner, the Better

When to quit smoking? How about right away, say the researchers.

They couldn't pinpoint the key time to quit during pregnancy. That's because most participants who smoked during pregnancy never stopped.

Pregnant or not, smoking isn't good for anyone. Bring a baby into the picture, and it's even more important to quit.

"Maternal smoking cessation before pregnancy can greatly reduce the risk of childhood asthma," writes Gilliland, who works at the University of Southern California's medical school.

Accurate Information?

Self-reports aren't always perfect, but Gilliland's team says it's unlikely that misreporting by participants would have changed the results.

To double-check, they called some of the grandmothers and found no pattern of incorrect reports about the grandmothers' smoking habits.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: WebMD Medical News: "Fewer Women Are Smoking During Pregnancy." WebMD Medical News: "Smoking in Pregnancy May Cause Genetic Harm." WebMD Medical Reference provided in collaboration with The Cleveland Clinic: "Pregnancy: Smoking During Pregnancy."