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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 sd 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 sd …