NEW HAVEN, Conn. - Pregnant women with asthma, if they are carrying a girl, are likely to have poorer lung function than if a boy is expected, a prospective study indicated.
In fact, lung function was about 10% poorer during pregnancy for asthmatic mothers carrying female fetuses compared with mothers carrying boys, reported Michael B. Bracken, Ph.D., M.P.H., and colleagues at Yale in the February issue of American Journal of Epidemiology.
The study, which involved 702 pregnant women with asthma, followed up on previous case reports and cross-sectional studies with similar findings. Peak expiratory flow was assessed in study participants at enrollment and at 21, 29, and 37 weeks' gestation.
Lung function improved from baseline in all participants until 29 weeks gestation, when it began to worsen. This pattern agreed with observations made by other researchers, and it could be caused by progesterone and free cortisol, which increase dramatically toward the end of pregnancy, binding to glucocorticoid receptors and thereby interfering with the effect of corticosteroids, the researchers speculated.
However, after adjusting for factors such as corticosteroid use, smoking, and emergency room visits for asthma, women with female fetuses had 9.9% lower measurements of lung function than women carrying males throughout the study period (95% confidence interval=19.4%-0.4%).
The authors speculated that testosterone, secreted by male fetuses from eight weeks onward, may relax the mother's bronchial tissue and inhibit response to histamines. "Hence, asthmatic women with male fetuses may experience a protective effect, particularly from the second trimester onward," the authors said.
The researchers also speculated that sex-specific factors produced by female fetuses might aggravate inflammation in mothers. However, the researchers emphasized that other conditions, such as smoking status and access to medical care, play a much greater role in the severity of a pregnant woman's asthma symptoms than the sex of her baby.
The women ranged in age from 14 to 48 and were recruited from clinics and private practices in southern New England between 1997 and 2000.
Asthma affects about 4% to 8% of pregnant women in the United states, the authors said. "Continued investigation of the effects of fetal sex on maternal asthma is warranted to clarify understanding of this widely prevalent disease," they concluded.