Background: Unwanted, intrusive thoughts of infant-related harm are a normal, albeit distressing experience for most new mothers. The occurrence of these thoughts can represent a risk factor for the development of obsessive compulsive disorder (OCD). As the early postpartum period represents a time of increased risk for OCD development, the transition to parenthood provides a unique opportunity to better understand OCD development. Aims: The purpose of this study was to assess components of cognitive behavioural conceptualizations of postpartum OCD in relation to new mothers’ thoughts of infant-related harm. Method: English-speaking pregnant women (n = 100) participated. Questionnaires were completed at approximately 36 weeks of gestation, and at 4 and 12 weeks postpartum. An interview to assess postpartum harm thoughts was administered at 4 and 12 weeks postpartum. Questionnaires assessed OC symptoms, OC-related beliefs, fatigue, sleep difficulties and negative mood. 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.