IMPORTANCE Many early life risk factors for childhood obesity are more prevalent among blacks and Hispanics than among whites and may explain the higher prevalence of obesity among racial/ethnic minority children. OBJECTIVE To examine the extent to which racial/ethnic disparities in adiposity and overweight are explained by differences in risk factors during pregnancy (gestational diabetes and depression), infancy (rapid infant weight gain, feeding other than exclusive breastfeeding, and early introduction of solid foods), and early childhood (sleeping <12 h/d, presence of a television set in the room where the child sleeps, and any intake of sugar-sweetened beverages or fast food). DESIGN Prospective prebirth cohort study. SETTING Multisite group practice in Massachusetts. PARTICIPANTS Participants included 1116 mother-child pairs (63% white, 17% black, and 4% Hispanic) EXPOSURE Mother's report of child's race/ethnicity. MAIN OUTCOMES AND MEASURES Age- and sex-specific body mass index (BMI) z score, total fat mass index from dual-energy x-ray absorptiometry, and overweight or obesity, defined as a BMI in the 85th percentile or higher at age 7 years. RESULTS Black (0.48 U [95% CI, 0.31 to 0.64]) and Hispanic (0.43 [0.12 to 0.74]) children had higher BMI z scores, as well as higher total fat mass index and overweight/obesity prevalence, than white children. After adjustment for socioeconomic confounders and parental BMI, differences in BMI z score were attenuated for black and Hispanic children (0.22 U [0.05 to 0.40] and 0.22 U [-0.08 to 0.52], respectively). Adjustment for pregnancy risk factors did not substantially change these estimates. However, after further adjustment for infancy and childhood risk factors, we observed only minimal differences in BMI z scores between whites, blacks (0.07 U [-0.11 to 0.26]), and Hispanics (0.04 U [-0.27 to 0.35]). We observed similar attenuation of racial/ethnic differences in adiposity and prevalence of overweight or obesity. CONCLUSIONS AND RELEVANCE Racial/ethnic disparities in childhood adiposity and obesity are determined by factors operating in infancy and early childhood. Efforts to reduce obesity disparities should focus on preventing early life risk factors.