Compared to their peers with non-Hispanic white mothers, children of Mexican-heritage mothers have higher average BMI and greater rates of obesity. The BMI of boys with Mexican-born mothers is higher relative to whites and children of U.S.-born Mexican mothers across early childhood, increasing sharply at about age 4.5 years. This divergence is driven by increases in the BMI of boys, as girls do not show the same growth. A number of measures, including descriptors of children's nutritional intake, lifestyle factors, and acculturation, do not explain the increased obesity rates among sons of Mexican mothers. Conclusion . Despite favorable perinatal health and weight, Mexican-American sons of foreign-born mothers show disadvantages in BMI that emerge close to the start of kindergarten.