 This study aims to prospectively determine mechanistic links between antibiotic exposure, disruption of the infant gut microbiome development, and obesity in children. Given the prevalence of early life antibiotic use and the health consequences of obesity, evidence of the proposed association will have important implications for public health and pediatric medicine. Currently in the United States, 32 of all laboring women receive intrapartum antibiotics that reach the unborn baby and almost 10 of all newborns are given broad-spectrum antibiotics immediately after birth. After discharge from the nursery, infants and young children have the highest rates of antibiotic prescribing of any age group in the US, with approximately 15 of all children receiving a systemic antibiotic by six months of age. Animal studies have linked antibiotic-induced alterations in the microbiome to adiposity, but large prospective human studies exploring associations between the entire spectrum of early life antibiotic exposure, gut microbiome development, and obesity have not been conducted. 