Pregnant women with asthma carrying a female fetus are more likely to experience worse symptoms of asthma than are those carrying a male fetus, research suggests.

Researchers studied 702 pregnant asthmatic women who were trained to assess their lung function, which they did for 10 days at selected points in their pregnancy. Lung function was recorded automatically and a large number of other factors that might influence mother’s asthma severity were also measured.

Throughout pregnancy, women who were pregnant with a male fetus enjoyed roughly 10 percent better lung function than women who were pregnant with a female fetus, according to a report of the study in the American Journal of Epidemiology.

“The mechanism for this putative effect of fetal sex on asthma severity during pregnancy remains uncertain,” Dr. Michael B. Bracken from Yale University in New Haven, Connecticut and colleagues acknowledge.

They speculate that testosterone, which male fetuses are known to secrete, may relax the mother’s airway tissue. “Hence, asthmatic women with male fetuses may experience a protective effect,” they offer. Alternatively, sex-specific factors related to the presence of a female fetus may promote inflammation.

Further study is needed to clarify the effects of fetal sex on maternal asthma, the team concludes, noting that asthma is one of the most common diseases found in pregnancy.

SOURCE: American Journal of Epidemiology, February 2006.