BACKGROUND: In recent studies a larger birth size has been shown to delay the timing of menarche. The mechanisms underlying this association are not clear, however, as birthweight is a predictor of body size in childhood, and a large body size is known to be associated with an early onset of menarche. METHODS: Data from a representative British cohort of 2547 girls born in 1946 who were followed prospectively throughout childhood were used. Information was available on prenatal characteristics, birthweight, height, weight and social circumstances during childhood, and on age at menarche. Random coefficients models were used to estimate the individual trajectories in height and body mass index (BMI) up to age 7 years. The parameters identified by these models were then included in Weibull survival models for the timing of menarche together with birthweight. RESULTS: Birthweight was found to positively influence height and BMI values at age 2 years, but not to affect their rates of change from age 2 to 7 years. Initial analyses showed low birthweight to be associated with an early onset of menarche, but after controlling for growth in infancy this effect was reversed, with girls who were heavy at birth reaching menarche earlier than others with similar infant growth. Rapid growth in infancy was also related to early pubertal maturation. The effects of birthweight and infant growth disappeared, however, when further controlled for growth from age 2 to 7 years. CONCLUSIONS: The effects of birthweight and growth in infancy on the timing of menarche seem to be mediated through growth in early childhood. These findings are consistent with the possibility that timing of menarche may be set in utero or early in life, although it may be modified by changes in body size and composition in childhood.