May 19 (HealthDay News) -- A mother's weight may have lasting effects not just on her own health but on the respiratory health of her children as well.

"Children with asthmatic parents are at an increased risk of asthma if the mother is overweight before pregnancy," said H.A. Smit, head of the department of prevention and health services research at the National Institute of Public Health and Environment in the Netherlands.

In fact, Smith and his fellow researchers found that the risk of asthma is 65 percent higher among the offspring of overweight mothers if one or both of the child's parents have a history of the disease.

Smit was to present the findings Tuesday at the American Thoracic Society's annual meeting in San Diego.

As many as 20 million Americans have been diagnosed with asthma, about 9 million of them children, according to the American Academy of Allergy, Asthma & Immunology. Despite advances in treatment, asthma is still responsible for about 5,000 deaths each year in the United States, it says.

Not all children born to parents with asthma go on to develop the airway disease. That happens about 40 percent of the time, the academy reports.

Because the exact causes of asthma are not clear, researchers have looked at a number of factors that might contribute to its development, including maternal smoking, the child's environment and more.

Smit's study sought to assess whether a mother's weight before pregnancy could affect a child's risk for asthma. The study included nearly 4,000 children, who were followed from birth to 8 years of age.

The mothers in the study averaged 30 years old, and almost 21 percent were overweight -- which the researchers defined as have a body mass index higher than 25 -- before becoming pregnant.

Children were considered to have asthma if their parents reported that they'd had at least one attack of wheezing or shortness of breath or had needed inhaled corticosteroids in the previous year. About 14 percent of the children had asthma by age 8.

The researchers adjusted the data to account for confounding factors, such as maternal education, mode of delivery, maternal smoking during pregnancy, duration of breast-feeding, birth weight and the child's current weight, according to Smit.

Although they found no association between maternal weight in children born to parents without asthma, children born to parents with asthma who also had an overweight mother had a 65 percent increased risk of developing asthma.

Though the study was not designed to determine why being overweight might affect a child's risk for asthma, Smit theorized that inflammation could be the connection between the conditions. That's because obesity can encourage inflammation, and inflammation is at the root of asthma.

Dr. Jennifer Appleyard, chief of allergy and immunology at St. John Hospital and Medical Center in Detroit, said that "we don't know exactly what causes or contributes to asthma, but it does look like there are some things that occur in utero that could affect the child later."

But, she said, it may not be the fact that mothers are overweight. It could be something that they're eating that's affecting their children. It's just not clear from this study, she said, adding that that more research needs to be done.

SOURCES: H.A. Smit, Ph.D., head, Department of Prevention and Health Services Research, National Institute of Public Health and Environment, Bilthoven, the Netherlands; Jennifer Appleyard, M.D., chief, allergy and immunology, St. John Hospital and Medical Center, Detroit; May 19, 2009, presentation, 105th International Conference, American Thoracic Society, San Diego