BACKGROUND: Studies have raised concern about an association between the use of selective serotonin reuptake inhibitors (SSRIs) during pregnancy and an increased risk of autism spectrum disorders in the offspring. METHODS: We conducted a cohort study of all singleton live births in Denmark from 1996 through 2005 (626,875 births), with follow-up through 2009. Using Danish population registries, we linked information on maternal use of SSRIs before and during pregnancy, autism spectrum disorders diagnosed in the offspring, and a range of potential confounders. We used a survival analysis of the time to diagnosis in the offspring with Poisson regression to estimate rate ratios of autism spectrum disorders according to maternal use of SSRIs. RESULTS: During 5,057,282 person-years of follow-up, we identified 3892 cases of autism spectrum disorder (incidence rate, 77.0 per 100,000 person-years). A total of 52 cases during 42,400 person-years of follow-up involved offspring of women who were exposed to SSRIs during their pregnancy (incidence rate, 122.6 per 100,000 person-years). As compared with no use of SSRIs both before and during pregnancy, use during pregnancy was not associated with a significantly increased risk of autism spectrum disorders (fully adjusted rate ratio, 1.20; 95% confidence interval [CI], 0.90 to 1.61). Among women who received SSRIs before pregnancy but not during pregnancy, the corresponding fully adjusted rate ratio was 1.46 (95% CI, 1.17 to 1.81). CONCLUSIONS: We did not detect a significant association between maternal use of SSRIs during pregnancy and autism spectrum disorder in the offspring. On the basis of the upper boundary of the confidence interval, our study could not rule out a relative risk up to 1.61, and therefore the association warrants further study.