Context: Rapid postnatal weight gain has been associated with subsequent increased childhood adiposity. However, the contribution of rapid weight gain during specific infancy periods is not clear.

Objective: We aimed to determine which periods of infancy weight gain are related to childhood adiposity and also to age at menarche in UK girls.

Design, Setting, and Participants: A total of 2715 girls from a prospective UK birth cohort study participated in the study.

Main Outcome Measures: Routinely measured weights and lengths at ages 2, 9, and 19 months were extracted from the local child health computer database. Body composition was assessed by dual-energy x-ray absorptiometry at age 10 yr, and age at menarche was assessed by questionnaire (categorized into three groups: <12.0, 12.0–13.0, and >13.0 yr).

Results: Faster early infancy weight gain between 0 and 2 months and also 2 to 9 months were associated with increased body fat mass relative to lean mass at age 10 yr and also with earlier age at menarche. Each +1 unit gain in weight sd score between 0 and 9 months was associated with an odds ratio (95% confidence interval) = 1.48 (1.27–1.60) for overweight (body mass index > 85th centile) at 10 yr, and 1.34 (1.21–1.49) for menarche at less than 12 yr. In contrast, subsequent weight gain between 9 and 19 months was not associated with later adiposity or age at menarche.

Conclusions: In developed settings, rapid weight gain during the first 9 months of life is a risk factor for both increased childhood adiposity and early menarche in girls.