Breastfeeding may reduce obesity risk, but this association could be confounded by breastfeeding families' characteristics. We investigated if body composition differs at birth among infants who were either exclusively breast- or formula-fed. We hypothesized the two groups would differ in body composition, even at birth, prior to their post-natal feeding experience. Healthy primiparous carrying singleton pregnancy were recruited at 15 weeks' gestation. PEA POD« measured body composition within 72 hours of delivery and infant feeding was prospectively captured. Out of the 1,152 infants recruited, 117 (10.2%) and 239 (20.7%) went on to be either exclusively breast- or formula-fed, respectively. Breastfed infants were heavier at birth, but their percentage fat mass (FM) was lower than that of exclusively formula-fed infants (covariate adjusted ▀ = -1.91 percentage points of FM; 95% CI -2.82 to -1.01). Differences in intra-uterine exposures, irrespective of early diet, may partly explain an infant's obesity risk.