Age at any full-term pregnancy and breast cancer risk.

The authors analyzed data from two multistate, population-based case-control studies to investigate the association between age at any full-term pregnancy (FP) and breast cancer risk. Study subjects included breast cancer cases aged 20-79 years identified from four statewide cancer registries and randomly selected controls interviewed from 1988 to 1996. Complete information on a comprehensive set of risk factors for breast cancer was available for 9,891 cases and 12,271 controls. The large number of subjects enabled simultaneous 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 indistinguishable from that for other FPs after the first. In this analysis, a modest and transient increase in breast cancer risk after childbirth was also observed. The relatively greater effect of age at first FP is consistent with the existence of a long-term effect of early first FP on the differentiation of mammary cells, causing them to become less susceptible to carcinogenesis.

Lactation and a reduced risk of premenopausal breast cancer.

BACKGROUND: The evidence of an association of lactation with a reduction in the risk of breast cancer among women has been limited and inconsistent. The effect of lactation appears to be confined to premenopausal women with a history of long lactation, but most studies of this relation have been limited in statistical power. We conducted a multicenter, population-based, case-control study with a sample large enough for us to describe more precisely the association between lactation and the risk of breast cancer. METHODS: Patients less than 75 years old who had breast cancer were identified from statewide tumor registries in Wisconsin, Massachusetts, Maine, and New Hampshire. Controls were randomly selected from lists of licensed drivers if the case subjects were less than 65 years old, and from lists of Medicare beneficiaries if they were 65 through 74 years old. Information on lactation, reproductive history, and family and medical history was obtained by means of telephone interviews. After the exclusion of nulliparous women, 5878 case subjects and 8216 controls remained for analysis. RESULTS: After adjustment for parity, age at first delivery, and other risk factors for breast cancer, lactation was associated with a slight reduction in the risk of breast cancer among premenopausal women, as compared with the risk among women who were parous but had never lactated (relative risk, 0.78; 95 percent confidence interval, 0.66 to 0.91); the relative risk of breast cancer among postmenopausal women who had lactated, as compared with those who had not, was 1.04 (95 percent confidence interval, 0.95 to 1.14). With an increasing cumulative duration of lactation, there was a decreasing risk of breast cancer among premenopausal women (P for trend < 0.001) but not among postmenopausal, parous women (P for trend = 0.51). A younger age at first lactation was significantly associated with a reduction in the risk of premenopausal breast cancer (P for trend = 0.003). As compared with parous women who did not lactate, the relative risk of breast cancer among women who first lactated at less than 20 years of age and breast-fed their infants for a total of six months was 0.54 (95 percent confidence interval, 0.36 to 0.82). CONCLUSIONS: There is a reduction in the risk of breast cancer among premenopausal women who have lactated. No reduction in the risk of breast cancer occurred among postmenopausal women with a history of lactation.