Results: Prenatal OC-related beliefs predicted postpartum OC symptoms, as well as harm thought characteristics and behavioural responses to harm thoughts. The severity of behavioural responses to early postpartum harm thoughts did not predict later postpartum OC symptoms, but did predict frequency and time occupation of accidental harm thoughts, and interference in parenting by intentional harm thoughts. Strong relationships between OC symptoms and harm thought characteristics, and concurrent sleep difficulties, negative mood and fatigue were also found. Conclusions: Findings provide support for cognitive behavioural conceptualizations of postpartum OCD and emphasize the importance of maternal sleep, fatigue and negative mood in the relationship between OC-related beliefs and maternal cognitive and behavioural responses to postpartum harm thoughts.