BACKGROUND: Maternal characteristics and childhood growth have been identified as risk factors for eating disorders. Most studies to date have been unable to investigate these factors prospectively while accounting for their interdependencies. We address this by investigating whether the association of maternal prepregnancy body mass index (ppBMI) with adolescent eating disorder behaviors can be explained by childhood growth and/or a concurrent environmental pathway captured by maternal eating habits. METHODS: We analyzed data from girls participating in the Avon Longitudinal Study of Parents and Children (ALSPAC), a prospective UK cohort. The study had information on parentally and self-reported eating disorder behaviors at age 13/14 years (n = 3,529), maternal ppBMI and eating habits at age 8, child's birth weight, BMI from age 7 to 12, pubertal development at 11, and relevant confounders. We quantified contributions of childhood growth and concomitant maternal eating habits to the association of maternal ppBMI with eating disorder behaviors in terms of interventional disparity effects for multiple mediators. RESULTS: Maternal prepregnancy underweight was negatively associated with eating disorder behaviors (-0.18; 95% confidence interval: -0.29, -0.06), whereas overweight/obesity had the opposite relationship (0.25; 0.18, 0.32). Both were nearly fully explained by childhood growth. CONCLUSIONS: Although maternal ppBMI is associated with developing eating disorders, its role needs to be understood in the context of childhood factors, in particular childhood growth. The relatively small size of the remaining associations, once growth factors are hypothetically equalized across levels of maternal ppBMI, suggests that childhood growth is a potential area for prevention.