Objective: To determine the influence of birth weight and postnatal weight gain on age at menarche.

Design, Setting, Participants: This was a prospective cohort study where girls from the West Australian Pregnancy (Raine) Cohort Study were followed prospectively since fetal life (18 weeks of pregnancy) to adolescence (12-14 yr).

Main Outcome Measure: Age at menarche.

Results: Growth status at birth was judged by expected birth weight ratio (EBW: a ratio of observed infant's birth weight over median birth weight appropriate for maternal age, weight, height, parity, infant sex, and gestational age). Postnatal growth status was judged by body mass index (BMI). Both EBW (P = 0.020) and BMI in childhood (8 yr of age) (P < 0.001) were associated with age at menarche. Menarche occurred earlier in girls with lower EBW and higher BMI.

Conclusions: We have demonstrated for the first time that both birth weight and weight gain in childhood are associated with age at menarche. Weight gain before birth and subsequent weight gain up to the age of 8 yr were found to have opposing influences on the timing of menarche. Lower expected birth weight ratio combined with higher BMI during childhood predicted early age at menarche and this relationship existed across normal birth weight and BMI ranges.