Age at first birth, parity, and breast cancer risk.

The roles of age at first full-term birth and parity as risk factors for breast cancer were examined as part of a large prospective cohort study of oral contraceptive use. Compared to women who first gave birth before age 20 years, women with a first birth between age 20 and 24 years had an SRR of 1.70, and women with first births between age 25 and 29 years and over age 29 had SRRs of 2.19 and 3.18, respectively. Increased incidence rates of breast cancer fell with increasing parity, although standardization for age at first birth diminished the importance of the former as an independent indicator of breast cancer risk.