Age at any full-term pregnancy and breast cancer risk

The authors analyzed data from two multistate, population-based case-contro l studies to investigate the association between age at any full-term pregn ancy (FP) and breast cancer risk. Study subjects included breast cancer cas es aged 20-79 years identified from four statewide cancer registries and ra ndomly selected controls interviewed from 1988 to 1996. Complete informatio n on a comprehensive set of risk factors for breast cancer was available fo r 9,891 cases and 12,271 controls. The large number of subjects enabled sim ultaneous adjustment of the covariates and efficient application of various modeling approaches. Overall, each 5-year increase in age at first FP was associated with an odds ratio of 1.07 (95% confidence interval (CI): 1.01, 1.13) for breast cancer. The corresponding estimates were odds ratio = 1.02 (95% CI: 1.00, 1.05) for age at second through ninth FPs. For age at last FP, the effect estimate (odds ratio = 1.01, 95% CI: 0.97, 1.06) was indisti nguishable from that for other FPs after the first. In this analysis, a mod est and transient increase in breast cancer risk after childbirth was also observed. The relatively greater effect of age at first FP is consistent wi th the existence of a long-term effect of early first FP on the differentia tion of mammary cells, causing them to become less susceptible to carcinoge nesis.