Analyses of program engagement can provide critical insight into how program involvement leads to outcomes. This study examines the relation between participant engagement and program outcomes in Family Foundations (FF), a universal preventive intervention designed to help couples manage the transition to parenthood by improving coparenting relationship quality. Previous intent-to-treat outcome analyses from a randomized trial indicate FF improves parental adjustment, interparental relationships, and parenting. Analyses for the current study use the same sample, and yield statistically reliable relations between participant engagement and interparental relationships but not parental adjustment or parenting. Discussion considers implications for FF and the difficulties researchers face when examining the relation between engagement and outcomes in preventive interventions.