The transition to parenthood is a stressful period for most parents as individuals and as couples, with variability in parent mental health and couple relationship functioning linked to children’s long-term emotional, mental health, and academic outcomes. Few couple-focused prevention programs targeting this period have been shown to be effective. The purpose of this study was to test the short-term efficacy of a brief, universal, transition-to-parenthood intervention (Family Foundations) and report the results of this randomized trial at 10 months postpartum. This was a randomized controlled trial; 399 couples expecting their first child were randomly assigned to intervention or control conditions after pretest. Intervention couples received a manualized nine-session (five prenatal and four postnatal classes) psychoeducational program delivered in small groups. Intent-to-treat analyses indicated that intervention couples demonstrated better posttest levels than control couples on more than two thirds of measures of coparenting, parent mental health, parenting, child adjustment, and family violence. Program effects on family violence were particularly large. Of eight outcome variables that did not demonstrate main effects, seven showed moderated intervention impact; such that, intervention couples at higher levels of risk during pregnancy showed better outcomes than control couples at similar levels of risk. These findings replicate a prior smaller study of Family Foundations, indicating that the Family Foundations approach to supporting couples making the transition to parenthood can have broad impact for parents, family relationships, and children’s adjustment. Program effects are consistent and benefit all families, with particularly notable effects for families at elevated prenatal risk.