Contemporary cognitive models of obsessive-compulsive disorder (OCD) posit that OC symptoms arise from negative interpretations of intrusive thoughts, which are derived from trait-like dysfunctional assumptions ("obsessive beliefs;" e.g., concerning overestimates of responsibility). Although correlational studies suggest that obsessive beliefs, negative interpretations of intrusions, and OC symptoms are interrelated, prospective studies evaluating the directional hypotheses implied in the cognitive model are lacking. In the present longitudinal study, 76 first time expecting parents were followed through the postpartum. Results indicated that the tendency to negatively interpret the presence and meaning of unwanted intrusive infant-related thoughts early in the postpartum period (3-4 weeks) mediated the relationship between pre-childbirth obsessive-beliefs and late postpartum (12 weeks) OC symptoms. Results are discussed in terms of their theoretical and treatment implications.