IMPORTANCE: Behavioral interventions to treat childhood obesity have had limited success. Primary prevention is desirable, but whether intervention effectiveness can be sustained is unknown. OBJECTIVE: To examine the effect of an intervention designed for the primary prevention of obesity and delivered through age 2 years on weight outcomes through age 9 years. DESIGN, SETTING, AND PARTICIPANTS: A longitudinal observation of a single-center randomized clinical trial comparing a responsive parenting intervention vs a home safety intervention (control) among primiparous mother-child dyads who completed the assessment at age 3 years with follow-up to age 9 years. All data were analyzed from January 21 to November 15, 2024. INTERVENTIONS: Research nurses conducted 4 home visits during infancy and research center visits at ages 1 and 2 years totaling less than 10 contact hours. The responsive parenting curriculum focused on feeding, sleep, interactive play, and emotion regulation. MAIN OUTCOMES AND MEASURES: The primary outcome is body mass index (BMI
calculated as weight in kilograms divided by height in meters squared) across 4 assessments from age 3 through 9 years, with the assessment of study group differences using repeated-measures analysis. A test for an interaction between sex and study group was planned. Secondary outcomes include BMI z scores and prevalence of overweight (BMI ≥85th to <
95th percentile) and obesity (BMI ≥95th percentile) at 5, 6, and 9 years. RESULTS: Of the 232 primiparous mother-child dyads (116 per group) (7 Asian [3%], 11 Black [5%], 1 Native Hawaiian or Other Pacific Islander [0.4%], 207 White [89%], and 6 children with other race and ethnicity [including Asian, Indian, Hispanic, Dominican, and other race
2.5%]
121 male children [52%]), 177 (76%) had anthropometric data at age 9 years. From ages 3 to 9 years, children in the responsive parenting group had a lower mean (SD) BMI than controls (16.64 [0.21] vs 17.07 [0.20]
absolute difference, -0.43
P = .049). Sex moderated this effect
female participants in the responsive parenting group had a lower mean (SD) BMI than female participants in the control group (16.32 [0.26] vs 17.32 [0.26]
absolute difference, -1.00
P = .007), with no group differences among male participants. Cross-sectional analyses revealed no differences in BMI z scores or prevalence of overweight or obesity at ages 5, 6, and 9 years between the responsive parenting group and the control group. CONCLUSIONS AND RELEVANCE: An early-life responsive parenting intervention resulted in lower BMI from age 3 to 9 years compared with a control intervention. This group difference was driven by effects on female participants, with differences appearing to dissipate over time. A life-course approach may be required to sustain the benefits of early-life responsive parenting interventions for obesity prevention. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03555331.