This study examined the efficacy of a multimodule parenting intervention, "My Baby & Me," that began prenatally and continued until children reached 2.5 years of age. The intervention targeted specific parenting skills designed to alter trajectories of maternal and child development. Of 361 high-risk mothers (193 adolescents, 168 adults) enrolled across 4 states, half were randomly assigned to the high-intensity (HI) home visitation coaching program (55 sessions), and half to a low-intensity (LI) condition that included monthly phone calls from a coach, printed informational materials, and community resource referrals. Videotaped observations of mother-child play were coded at 5 time points for multiple maternal and child behaviors and skills. Compared to mothers in the LI group, mothers in the HI group showed higher levels of contingent responsiveness, higher quality verbal stimulation, and more verbal scaffolding by 30 months, with higher levels of warmth and greater decreases in physical intrusiveness and negativity when their children were 24 months. By 30 months, children in the HI group showed more rapid increases and higher levels of engagement with the environment, expressive language skills, and social engagement, as well as more complex toy play and fewer problem behaviors than those in the LI group. Gains in maternal responsive behaviors mediated the effects of the intervention on child outcomes. Results were comparable for adolescent and adult mothers. A strong theoretical framework, consistent focus on maternal responsiveness, high dosage, and trusting relationships with coaches are thought to explain the positive outcomes.