The objectives of this study were: (1) to examine Composite International Diagnostic Interview (CIDI) period prevalence and comorbidity for depression and anxiety disorder in a cohort of women assessed during the first 6-8 months postpartum and (2) to examine the benefits of combining the Edinburgh Postnatal Depression Scale (EPDS) with a simple "interval symptom" question to optimize screening postpartum. Women aged over 18 (N=1,549) were assessed during late pregnancy and reviewed at approximately 2, 4, and 6-8 months postpartum using the EPDS and an "interval symptom" question. The latter asked about any depressive symptoms in the interval since the last EPDS. Women who scored >12 on the EPDS and/or positive on the "interval symptom" question were then administered the CIDI. A further 65 randomly selected women that screened negative were also administered the CIDI. Loss to postnatal follow-up was very significant, and returns rates were inconsistent across the three postnatal time points. Almost 25% of those who screened positive did not complete a CIDI. For screen-positive status, a total of 314 (24.4%) of those that returned questionnaires (N=1,289) screened positive at least once across the 6- to 8-month interval. Of these, 79 were lost to follow-up; thus, 235 (74.8%) completed a CIDI. In this group, 34.7% had been positive both on the EPDS and the "interval" question, 15.9% on the EPDS alone, and 49.4% on the "interval" question alone. For the CIDI diagnosis and estimated 6- to 8-month period CIDI prevalence, among those 235 women who screened positive and completed a CIDI, 67.2% met the criteria for a CIDI diagnosis, as did 16.9% of those who screened negative. The breakdown in CIDI diagnoses in the 235 women was 32.8% major depression (± anxiety disorder); 26.4% minor depression alone; and 8.1% with a primary anxiety disorder (approximately half with minor depression). Put another way, 20.4% of these women had an anxiety disorder (approximately two thirds with comorbid depression) and 37.7% of women with a major depressive episode (MDE) had a comorbid anxiety disorder. The estimated 6- to 8-month prevalence rate for a CIDI diagnosis of anxiety or depression (major or minor) was 29.2% (95% CI 26.7%-31.7%). The use of the "interval symptom" question alone was 1.7 times more likely to identify positive CIDI cases than the EPDS alone. Almost 40% of postnatal women with a diagnosis of MDE have a comorbid diagnosis of anxiety disorder. The estimated 6- to 8-month period prevalence for CIDI cases of anxiety and depression was 29.2%. Screening for anxiety and depression using the EPDS alone was associated with a lesser capacity to identify CIDI caseness than a simple "interval symptom" question (for the 2 months prior) which almost doubled the yield. This paper demonstrates that combining the EPDS with the "interval symptom" question improves detection of CIDI caseness.