There are few published studies of new mothers' experiences of intrusive thoughts of harm related to the newborn. Evidence-based information about the normal phenomenology of intrusive thoughts of harm related to the newborn is needed to facilitate appropriate clinical decision-making. The objective of this project was to assess the phenomenology, prevalence, correlates, and behavioural sequelae of maternal thoughts of harm related to the newborn. One hundred women were recruited during pregnancy. Participants were assessed prenatally and at 4 and 12 weeks postpartum using questionnaires and a semi-structured interview about unwanted thoughts of harm related to the newborn. Postpartum intrusive thoughts of accidental harm to the infant were universal, and close to half of the sample reported unwanted thoughts of intentionally harming their infant. Compared with intentional harm thoughts, accidental harm thoughts were more frequent and more time consuming, but less distressing. High parenting stress and low social support predicted the occurrence of thoughts of intentional harm. Little evidence of an association between these thoughts and aggressive parenting was found. Unwanted intrusive thoughts of harming one's infant are a relatively normative experience during the early postpartum period, particularly in association with greater parenting stress and low social support.