OBJECTIVE: To estimate the frequency of severe asthma exacerbations in pregnant women and to estimate whether there is an association with adverse perinatal outcomes. METHODS: Asthma exacerbations were evaluated in 146 women who were enrolled in a prospective cohort study of asthma and pregnancy. A severe exacerbation was defined as a hospital admission, emergency department presentation, or unscheduled doctor visit for asthma or a course of oral corticosteroids. Women were classified as having mild (n = 63), moderate (n = 34), or severe (n = 49) asthma. RESULTS: Severe exacerbations occurred in 8% (95% confidence interval [CI] 1.3-14.6%) of women with mild asthma, 47% (95% CI 30.3-63.8%) of women with moderate asthma, and 65% (95% CI 52-78.6%) of women with severe asthma at a mean gestational age of 25.1 +/- 0.9 (range 9-39) weeks of gestation. Among women who had severe exacerbations, there were 2 male stillbirths (P = .102) and a significantly increased rate of male low birth weight (P = .03). Maternal age, lung function, body mass index, gravidity, and parity were not different between women who did or those who did not have a severe exacerbation. Maternal pregnancy weight gain was significantly lower in women who had a severe exacerbation (P = .039). Forty-three percent of severe exacerbations occurred in winter, 34% were associated with self-reported viral infection, and 29% with nonadherence to inhaled corticosteroid medication. CONCLUSION: The exacerbation rate among pregnant women with asthma is high and associated with poor outcomes for the male fetus. Improvements in asthma management to prevent severe exacerbations may lead to a better outcome for both mother and baby. LEVEL OF EVIDENCE: II-2.