Unwanted, intrusive thoughts of actively harming one’s infant may be experienced by as many as 50% of first-time mothers, and are extremely upsetting to the women who experience them. Currently, little is known about these thoughts. Consequently, there is little information to guide women who experience them, their health-care professionals, or the child-protection system. There is no clear evidence upon which to base the best response when a woman discloses that she is experiencing unwanted, intrusive thoughts of harm pertaining to her infant. Protection of the infant is paramount in all cases. If intrusive thoughts of harm are predictable harbingers of violence, then acting to protect the infant is appropriate and necessary. Conversely, if (as appears to be the case) postpartum thoughts are a common and normative postpartum experience, and not indicative of danger to the infant, then taking dramatic steps is unnecessary and may result in serious negative consequences for the infant, the mother’s mental health, and the mother-infant relationship. This research will be the first large-scale, interview-based, prospective assessment of the relationships among maternal thoughts of infant-related harm, OCD, and maternal aggression. This study represents an important step towards the development of evidence-based health services for women who report thoughts of harm related to their infant.