Objectives: To examine the associations of antibiotic exposures during the first 2 years of life and the development of body mass over the first 7 years of life.

Design: Longitudinal birth cohort study.

Subjects: A total of 11 532 children born at 2500 g in the Avon Longitudinal Study of Parents and Children (ALSPAC), a population-based study of children born in Avon, UK in 1991-1992.

Measurements: Exposures to antibiotics during three different early-life time windows (<6 months, 6-14 months, 15-23 months), and indices of body mass at five time points (6 weeks, 10 months, 20 months, 38 months and 7 years).

Results: Antibiotic exposure during the earliest time window (<6 months) was consistently associated with increased body mass (+0.105 and +0.083 s.d. unit, increase in weight-for-length Z-scores at 10 and 20 months, P<0.001 and P=0.001, respectively; body mass index (BMI) Z-score at 38 months +0.067 s.d. units, P=0.009; overweight OR 1.22 at 38 months, P=0.029) in multivariable, mixed-effect models controlling for known social and behavioral obesity risk factors. Exposure from 6 to 14 months showed no association with body mass, while exposure from 15 to 23 months was significantly associated with increased BMI Z-score at 7 years (+0.049 s.d. units, P=0.050). Exposures to non-antibiotic medications were not associated with body mass.

Conclusions: Exposure to antibiotics during the first 6 months of life is associated with consistent increases in body mass from 10 to 38 months. Exposures later in infancy (6-14 months, 15-23 months) are not consistently associated with increased body mass. Although effects of early exposures are modest at the individual level, they could have substantial consequences for population health. Given the prevalence of antibiotic exposures in infants, and in light of the growing concerns about childhood obesity, further studies are needed to isolate effects and define life-course implications for body mass and cardiovascular risks.