Breast cancer is the most common female cancer globally. Greater breast density, i.e. the amount of fibro-glandular tissue seen on a mammogram or MRI of the breast, is a strong and highly common risk factor for breast cancer in the general population. The magnitude and consistency of the relationship between breast density and breast cancer risk make this the most significant target for breast cancer prevention. Epidemiologic data consistently report that higher birth weight (suggesting more rapid prenatal growth) and rapid height growth at puberty strongly predict later life breast cancer risk. Pubertal development also predicts breast cancer risk, with earlier age at menarche a strong risk factor for breast cancer. Sex steroids and growth factors regulate height growth and pubertal development as well as breast fibro-glandular tissue proliferation, suggesting that common mechanisms underlie growth, puberty, breast density and cancer risk. We will utilize data from two of the most comprehensively characterised pregnancy cohorts internationally. Together, the Avon Longitudinal Study of Parents and Children (ALSPAC) and the Western Australian Pregnancy Cohort (Raine) Study provide one of the few opportunities internationally to quantify how pre and postnatal growth, diet and endogenous and exogenous breast carcinogens impact on young adult breast density. Cohort strengths include: 1) large sample size (500 MRI-breast density measurements in ALSPAC; 600 to be collected in Raine); 2) existing repeat measures of pre and postnatal growth, adiposity, sex hormones and growth factors through to young adulthood; and 3) existing repeat measures of diet from infancy through to adulthood.