Specific Aim 2: To assess the comparative effects of different types, timing, and amount of antibiotics used during the first two years of life on the rates and patterns of childhood growth during the first 5 years of life. Hypothesis: There will be a "dose-response" relationship between the number of antibiotic courses given during the first 2 years of life and subsequent growth trajectories of children in a pattern that increases children's risk of later overweight and obesity. This relationship will be strongest for broad-spectrum antibiotics prescribed in the first 6 months of life.