Reproductive factors, exogenous female hormones, and colorectal cancer by subsite.

The associations between reproductive factors, exogenous hormones, and colorectal cancer were examined among female subjects in a population-based case-control study in Sweden. The study was performed in Stockholm in 1986-88, and included 299 cases and 276 controls. There was little evidence that age at first birth, number of months of breast feeding, age at menarche, or age at menopause influenced the risk of colon or rectal cancer. However, the results indicate that postmenopausal hormone-replacement therapy might reduce the risk of colorectal cancer (age-adjusted relative risk [RR] = 0.4, 95 percent confidence interval [CI] = 0.2-0.9). Compared with nulliparous women, women with at least four births were at reduced risk for colon cancer (RR = 0.5, CI = 0.2-1.2) but not rectal cancer (RR = 1.0, CI = 0.4-2.6). However, no trend across increasing parity was observed. Adjustments for diet, body mass, and physical activity had little influence on the results.