Women with asthma who are carrying a female fetus are more likely to experience worse asthma symptoms than asthmatic women carrying a male fetus, researchers at Yale School of Medicine report in the February issue of American Journal of Epidemiology.

“This is one of the first and largest studies to investigate the effect of fetal sex on the severity of the mother’s asthma, and one of the largest to investigate the effect of fetal sex on any disease of the mother,” said senior author Michael B. Bracken, Susan Dwight Bliss Professor of Epidemiology in the Department of Epidemiology and Public Health at Yale School of Medicine.

The researchers monitored 702 pregnant women throughout southern New England who were trained to assess their lung function for 10-day intervals at selected points in pregnancy.

Lung function and a large number of other factors that might influence severity of the mother’s asthma were recorded automatically.

Asthma worsened in mothers with either male or female fetuses until about 30 weeks gestation, after which there was an improvement in lung function. However, throughout pregnancy, mothers with a male fetus had 10 percent better lung function.

“This difference due to sex is potentially important but needs to be placed in the context of other factors which have a greater impact on the severity of mother’s asthma, including inadequate medical management of asthma symptoms, and whether the mother was a smoker or not,” said Bracken, who also co-directs the Yale Center for Perinatal, Pediatric and Environmental Epidemiology.

The authors speculate that testosterone, secreted by the male fetuses, may relax the mother’s bronchial tissue and inhibit response to histamines. Other sex-specific factors excreted by female fetuses may aggravate inflammation in mothers. Bracken said more research is needed to test these hypotheses.

Asthma is one of the most common diseases associated with pregnancy. An upcoming study by the authors to be published this spring, shows that eight to nine percent of pregnant women have a history of asthma.