Background: Women with bipolar disorder (BD) are at high risk of mania/psychosis following childbirth. The risk factors for these episodes remain poorly understood and prospective studies are rare. Here, we examine whether mood episodes occurring within pregnancy predict postpartum recurrence in women with BD using a prospective design.\ud\udMethod: 128 women with DSM-5 BD were followed from week 12 of pregnancy (baseline) to 12-weeks postpartum. Semi-structured interviews, supplemented by clinician questionnaires and case-note review, assessed lifetime psychiatric history at baseline, and perinatal psychopathology at two follow-up assessments: third-trimester of pregnancy and 12-weeks postpartum.\ud\udResults: Postpartum follow-up data were obtained for 124/128 (97%) women [98 bipolar I disorder/schizoaffective-BD (BD-I/SA-BD group) and 26 bipolar II disorder/other specified BD and related disorder (BD-II/BD-OS group)]. Perinatal recurrence was high in both diagnostic groups (57% and 62% respectively). Women with BD-I/SA-BD were significantly more likely to experience mania/psychosis within 6 weeks postpartum (23%, n=22/96) compared to those with BD-II/BD-NOS (4%, n=1/25; p=0.042). In BD-I/SA-BD, mania/psychosis in pregnancy significantly elevated risk of mania/psychosis postpartum compared to remaining well (RR 7.0, p