Birth size in relation to age at menarche and adolescent body size: implications for breast cancer risk

Early age at menarche and tallness are associated with increased risk for breast cancer, and emerging evidence suggests that body size at birth also is positively associated with breast cancer risk in adulthood. We have explored whether this effect of birth size could be mediated by influencing age at menarche or body height during adolescence. Information on age at menarche and adolescent height among 3,343 Norwegian girls 13 to 19 years of age, born at term and whose mothers had no history of gestational disease was individually linked to information on birth weight and length recorded in the national Medical Birth Registry. We related quintiles of birth size, adjusted for length of gestation, to age at menarche and adolescent height and weight, using general linear modeling and Cox regression analyses. In a subsample of the population, we also had information on maternal age at menarche, as well as adult height and body mass index in both parents. We explored whether parental factors confounded the association between perinatal predictors and adolescent outcomes in the daughters. We found that age at menarche tended to occur earlier with decreasing birth weight (p for trend = 0.03), and girls who were thin at birth (i.e., in the lowest quintile of ponderal index) had an earlier age at menarche than other girls (p = 0.02). Weight and length at birth were strongly and positively related to tallness during adolescence (p for trend < 0.001), but ponderal index, indicating obesity at birth, was negatively associated with adolescent height (p for trend = 0.002). Adjustment for parental height, body mass index and maternal age at menarche, in the subsample where this information was available, indicated no substantial confounding by these parental factors. Our results suggest that a positive association between birth size and adult breast cancer is not likely to be mediated by early age at menarche, but possibly by increased body height in adolescence.