Background and objective - Several studies have reported an increased risk of breast cancer in twins compared with singletons (1-3). The risk appears to be larger in dizygotic compared to monozygotic twins and singletons. Dizygotic pregnancies may have two placentas or one larger placenta which may present opportunities for larger estrogen exposures than monozygotic pregnancies (4). Early age at menarche is also linked to an increase in the risk of breast cancer later in life (5). We would like to explore whether twinship, zygosity, and the sex of the co-twin are associated with timing of puberty in girls enrolled in the Avon Longitudinal Study of Parents and Children.

Statistical analyses - Bivariate analyses will be conducted to test maternal prenatal characteristics. These will be retained in the analyses as confounders if they are associated to the study outcomes. We will construct parametric survival models to estimate the adjusted difference in median age of entry into stages >=2 of breast and pubic hair development and menarche associated with the predictors under examination in comparison groups. Analyses will be conducted using PROC LIFEREG (6) in SAS. Human subject protection was assessed and approved by the ALSPAC Law and Ethics Committee, the Local Research Ethics Committees, and the CDC Institutional.

Preliminary analyses - Of the ~ 4000 girls enrolled in ALSPAC who responded to the Growing and Changing Questionnaire I identified 82 co-twin girls with valid puberty data. Of these: 1) 44 girls (22 pairs) have a female co-twin with valid puberty data; 2) another 6 girls have a female co-twin with invalid puberty data; and 3) 31 have girls a male co-twin. The number of co-twin girls with puberty data appears to be adequate to conduct the analyses proposed.

Data needs from ALSPAC - 1) birth weight of twin births; and 2) zygosity of the twins.