We compare contemporaneous and retrospective reports of cohabitation among unmarried mothers in the Fragile Families and Child Wellbeing survey (N = 2,524). We find that (a) many mothers revise their reports of whether they cohabited at the time of the birth of their child and (b) revisions in reports are systematically related to individuals' characteristics and the quality and trajectory of parents' relationships. These results have important implications for analyses of determinants and consequences of family structure using these data and, potentially, any retrospective data on relationships and family structure.