OBJECTIVE: To estimate the incidence of pregnancy-associated breast cancer during different calendar periods and when pregnancy-associated breast cancer was diagnosed in relation to delivery. METHODS: This was a population-based cohort study using data from Swedish registers between 1963 and 2002, encompassing women aged 15-44 years at the date of breast cancer diagnosis. Outcome measures included incidence of pregnancy-associated breast cancer per 100,000 deliveries, the proportion of pregnancy-associated breast cancer among all breast cancers, and observed-to-expected rates. RESULTS: Between 1963 and 2002, 1,161 cases of pregnancy-associated breast cancer among a total of 16,620 breast cancers were identified in women aged 15 to 44 years. The incidence of pregnancy-associated breast cancer increased from 16.0 to 37.4 per 100,000 deliveries comparing the first and last calendar periods under study. During pregnancy, the overall incidence was 2.4 per 100,000 deliveries; the incidence during the first and second year after delivery was 10.6 and 15.0 per 100,000 deliveries, respectively. Fewer pregnancy-associated breast cancers than expected were diagnosed during pregnancy and the first 6 months after delivery. Thereafter, there was no difference between observed compared with expected number of breast cancers. CONCLUSION: The incidence of pregnancy-associated breast cancer increased during the study period, partly caused by a trend of postponement of childbearing to an older age. The present findings suggest that breast cancer is underdiagnosed during pregnancy and lactation. LEVEL OF EVIDENCE: III.