Maternal risk of breast cancer and birth characteristics of offspring by time since birth

We examined the association between birth characteristics of offspring and the subsequent maternal risk of breast cancer in a population-based cohort of 998,499 women; 13 to 48 years of age at entry. There were 9,495 incident cases of breast cancer during 12.8 million person-years of follow-up among these women. Compared with mothers of singleton infants, mothers having a multiple birth had an increased risk of breast cancer in the first 5 years after a birth (relative risk (RR) = 1.8; 95% confidence interval (CI) = 1.1 -2.8). The risk for mothers having a heavy weighted child (>3.75 kg), as co mpared with a child of light weight (less than or equal to 3 kg), was also slightly increased (RR = 1.2; 95% CI = 0.9-1.5). This latter effect was pri marily due to an increased incidence of tumors larger than 2 cm at diagnosi s (RR = 1.4; 95% CI = 0.9-1.9). Our findings are compatible with the hypoth esis that the hormonal level during pregnancy influences the risk of breast cancer in the early years after delivery.