Relationships between birthweight and future obesity risk remain unclear. To assess associations between birthweight and later obesity in a nationally representative cohort of early school-aged children. We used linear and logistic regression to evaluate 10186 term- or preterm children in the Early Childhood Longitudinal Study-Kindergarten Cohort 2011 for relationships between birthweight and later obesity and change in BMI z-score from kindergarten-to-second grade. All analyses were adjusted for sex, race/ethnicity, parental education and household income. Compared to children born normal birthweight (NBW), high birthweight (HBW) term children and large-for-gestational-age (LGA) preterm children had significantly greater BMI z-scores from kindergarten-to-second grade (p<0.001). Term children born HBW had higher odds of obesity by kindergarten (adjusted odds ratios [aOR] 1.91, p<0.0001). Among preterm children, odds of obesity was higher among LGA children starting in first grade (aOR 2.34, p<0.05) and among small-for-gestational age children in second grade (aOR 2.26, p<0.05). Compared to NBW children, HBW children had greater change in BMI z-score between kindergarten-first grade (p<0.01). High birthweight term and LGA preterm children had increased adjusted odds of obesity in school-age compared to their NBW counterparts. Physicians may provide counselling early in life for families of large infants to help prevent future obesity.