Parity and decreased use of oral contraceptives as predictors of asthma in young women

Background Asthma is more prevalent among males in childhood, but females report higher rates in adulthood. The reasons are unknown; although it has been hypothesized that hormonal factors may explain this sex-dependent risk of adult-onset asthma. Objective To determine whether a woman's reproductive history or use of oral contraceptives is associated with adult-onset asthma. Methods In 1991-1993, we surveyed 681 women aged 29-32 years randomly sampled from participants first surveyed at age 7 years by the 1968 Tasmanian Asthma Survey, a study of all children born in 1961 and attending school. Current asthma was defined as reporting asthma or wheezy breathing in the past 12 months. Results In women who did not have asthma or wheezy breathing by age 7 years, 13% had current asthma. The risk of current asthma in these who were parous increased with the number of births (odds ratio (OR) 1.50 per birth, 95% confidence interval (CI) 1.01-2.23 P=0.04) while women with one birth were at a lower risk than nulliparous women (OR 0.46 95% CI 0.2-1.06, P=0.07). Independent of parity, the risk decreased by 7% (95% CI 0-13%) per year of oral contraceptive pill use in all women. In women who did have asthma or wheezy breathing by age 7 years, neither reproductive history nor oral contraceptive pill use predicted current asthma. Conclusion Our observation that parity and decreased oral contraceptive use predict asthma in women, is consistent with the hypothesis that the asthma that develops after childhood is in part a response to endogenous and exogenous female hormones. This may be due to alterations of cytokine responses by the pregnant state, triggering adult-onset asthma in women.