OBJECTIVES:Interventions to prevent postpartum common mental disorders (PCMD) among unselected populations of women have had limited success. The aim was to determine whether What Were We Thinking (WWWT) a gender-informed, psychoeducational programme for couples and babies can prevent PCMD among primiparous women 6 months postpartum. DESIGN:Cluster-randomised controlled trial. SETTING:48 Maternal and Child Health Centres (MCHCs) from 6 Local Government Areas in Melbourne, Australia were allocated randomly to usual care (24) or usual care plus WWWT (24). PARTICIPANTS:English-speaking primiparous women receiving primary care at trial MCHCs were recruited to the intervention (204) and control (196) conditions. Of these, 187 (91.7%) and 177 (90.3%) provided complete data. INTERVENTION:WWWT is a manualised programme comprising primary care from a trained nurse, print materials and a face-to-face seminar. MAIN OUTCOME MEASURES:Data sources were standardised and study-specific measures collected in blinded computer-assisted telephone interviews at 6 and 26 weeks postpartum. The primary outcome was PCMD assessed by Composite International Diagnostic Interviews and Patient Health Questionnaire (PHQ) Depression and Generalised Anxiety Disorder modules. RESULTS:In intention-to-treat analyses the adjusted OR (AOR) of PCMD in the intervention compared to the usual care group was 0.78 (95% CI 0.38 to 1.63, ns), but mild to moderate anxiety symptoms (AOR 0.58, 95% CI 0.35 to 0.97) and poor self-rated health (AOR 0.46, 95% CI 0.22 to 0.97) were significantly lower. In a per protocol analysis, comparing the full (three component) intervention and usual care groups, the AOR of PCMD was 0.36, (95% CI 0.14 to 0.95). The WWWT seminar was appraised as salient, comprehensible and useful by >85% participants. No harms were detected. CONCLUSIONS:WWWT is readily integrated into primary care, enables inclusion of fathers and addresses modifiable risks for PCMD directly. The full intervention appears a promising programme for preventing PCMD, optimising family functioning, and as the first component of a stepped approach to mental healthcare. TRIAL REGISTRATION NUMBER:ACTRN12613000506796; Results.