Although family systems theory posits reciprocal causality between subsystems of the family, such as intimate partner violence exacerbating harsh parenting and vice versa, longitudinal studies with cross-lagged models have been used infrequently to test these principles. As guided by the spillover model, this study examined bidirectional associations between couple dysfunction, parent–child aggression risk, and child functioning across the transition to parenthood to determine whether and how disruptions in one subsystem relate to problems in other family subsystems. Participants were 201 first-time mothers and 151 fathers from a diverse community sample, evaluated during pregnancy, and reassessed two more times through their child's first 18 months of life. Individual and dyadic path model results indicate bidirectional spillover effects between parent–child aggression risk and child functioning for both mothers and fathers, and spillover from parent–child aggression risk to couple dysfunction for mothers but not fathers. However, limited spillover effects were identified between couple functioning and child adjustment, in contrast to previous work. Findings suggest that spillover can happen reciprocally and early in the family, supporting transactional models of behavior and highlighting the need for early family level intervention.