The first-onset affective episode requiring inpatient treatment in the postpartum period can be a marker of bipolar disorder, but it is unknown whether milder postpartum affective episodes are also indicators of underlying bipolarity. Therefore, we aimed to study whether women with a nonpsychotic postpartum affective episode treated with antidepressants have an increased risk of bipolar disorder.A register-based cohort study was conducted in Denmark of 122,622 parous women without psychiatric history who received a first-time antidepressant prescription during 1997-2012. We compared women with a first-time antidepressant prescription, which was our indicator of a first-onset affective disorder, within 1 year postpartum to women with a first-time antidepressant prescription outside the postpartum period. Our outcome was psychiatric contact for bipolar disorder (ICD-10 criteria) during follow-up, and we estimated hazard ratios using Cox regressions.The risk of bipolar disorder among women with a postpartum affective episode was higher than that in women with an affective episode outside the postpartum period. The risk of bipolar disorder was 1.66 (95% CI, 1.12-2.48) for postpartum antidepressant monotherapy and 10.15 (95% CI, 7.13-14.46) for postpartum antidepressant therapy plus a subsequent prescription for anxiolytics when these therapies were compared to antidepressant monotherapy outside the postpartum period.First-onset nonpsychotic postpartum affective disorder can be a marker of underlying bipolarity. Women who fill an antidepressant prescription following childbirth should be asked about hypomanic or manic symptoms and monitored long term. Clinically, when antidepressant monotherapy is ineffective or the individual woman experiences persistent and concerning symptoms, health professionals should consider a possible bipolar spectrum disorder.