Recent studies have found that asthmatic women pregnant with a female fetus reported more symptoms and had slightly lower lung function than women pregnant with a male fetus. In order to further investigate this association, we studied the effect of fetal sex on maternal asthma exacerbations and the use of asthma medications during pregnancy. A large cohort of pregnant asthmatic women and their babies was reconstructed between 1990 and 2002 from the linkage of three administrative databases of the Canadian province of Quebec. Asthma exacerbations were defined as a filled prescription of oral corticosteroids, an emergency department visit, or a hospitalization for asthma. Women pregnant with a female fetus were compared to women with a male fetus with respect to their rate of asthma exacerbation, their weekly doses of inhaled short-acting beta(2)-agonists (SABA), and their daily dose of inhaled corticosteroids (ICS) during pregnancy. Logistic and linear regression models were used to obtain effect measures adjusted for several potential confounders such as asthma severity and control prior to pregnancy. The cohort included 5529 pregnancies with a single female fetus and 5728 pregnancies with a single male fetus. No significant differences were found between mothers of a female and male fetus as to the occurrence of asthma exacerbations (adjusted rate ratio=1.02; 95% CI: 0.92-1.14), the daily dose of ICS (adjusted mean difference (AMD): 2.46 microg; 95% CI: -4.01 to 8.93), and the weekly dose of SABA (AMD: 0.004 dose; 95% CI: -0.23 to 0.24). Based on the results, we conclude that fetal gender is unlikely to affect maternal asthma during pregnancy to the point where acute care and medications are more often required among women pregnant with a female fetus.