Girls whose mothers smoked heavily during pregnancy are more likely to have asthma symptoms at age 14, a study from Australia shows. However, the researchers found no such link between a mother’s smoking and boys’ asthma risk.

The gender difference may be because boys mature at different rates than girls, or maternal smoking could be more damaging to the lungs of female fetuses, Dr. Rosa Alati of the University of Queensland and her colleagues suggest.

Young children whose parents smoke at home are known to be at greater risk of asthma and wheezing, the researchers note in a report in Epidemiology, while mothers who smoke during pregnancy may also increase their child’s risk of having asthma symptoms early in life.

But there is little information, they add, on whether this increased asthma risk persists past early childhood.

To investigate, the researchers analyzed the results of a study including nearly 4,000 children born between 1981 and 1984.

At age 14, the girls were nearly twice as likely to have asthma symptoms if their mothers smoked at least 20 cigarettes a day during pregnancy. The risk was slightly greater if mothers also smoked heavily during the girls’ early infancy. However, the researchers found, in utero exposure to cigarette smoking had a stronger effect than exposure after birth.

Maternal smoking had no impact on boys’ asthma risk, the researchers found. This may because boys and girls are at different stages of pubertal development at age 14, they note.

For example, while girls in the study who had not yet menstruated exhibited no increased asthma risk linked to maternal smoking, those who were having their periods were at greater risk. Another possibility is that prenatal smoking exposure is more harmful to the lungs of female fetuses.

While the increased risks for girls were seen consistently at all ages, Alati and her team note, other studies have not found this gender difference, making the findings “difficult to interpret.”

SOURCE: Epidemiology, March 2006.