The objectives of the proposed study are to assess the effects of different types, timing, and amount of antibiotic use in the first two years of life with (Specific Aim 1) body mass index (BMI) and obesity at ages 5 and 10 years and (Specific Aim 2) growth trajectories to age 5 years. In Specific Aim 3, the investigators will address how big the effects of antibiotics on obesity are within subgroups of the population, such as different racial/ethnic groups and whether the child's mother got antibiotics while she was pregnant.