Reproductive and menstrual factors and risk of colorectal cancer.

The relation between reproductive and menstrual factors and the risk of colorectal cancer was investigated in a case-control study conducted in Northern Italy on 279 women with colon cancer, 153 with rectal cancer and 386 controls admitted to hospital for a wide spectrum of acute, nonneoplastic, nondigestive disorders. Compared with nulliparous women, the relative risks (RR) of colon cancer were 1.1 for one and two births, 1.3 for three or four, and 1.1 for five or more. Corresponding values for rectal cancer were 0.9 for one, 1.1 for two, 1.4 for three or four, and 0.8 for five or more births. No significant association emerged with number of abortions, and no consistent pattern of trends for both colon and rectum cancer was observed in relation to age at first or last birth. Women whose menarche occurred at age 15 or over were at significantly lower risk of colon cancer (RR 0.5, 95% confidence interval = 0.3-0.9), and the point estimate was below unity, though nonsignificantly, for rectal cancer too (RR = 0.7). There was no relation between age at menopause, duration of menstrual cycles, and cancers of the colon and rectum. The present study produced no evidence that reproductive factors are related to colorectal cancer in this population. With this sample size, it was possible to exclude relative risks below 0.8 for colon and 0.7 for rectal cancer among multiparous women versus nulliparous ones. These findings are thus inconsistent with the epidemiological evidence on reproductive and menstrual factors and breast cancer in this and other populations, with the sole potential exception of the reduced risk of colorectal cancer among women whose menarche occurred at age 15 or over.