The influence of parity and exogenous female hormones on the risk of colorectal cancer.

Evidence that female sex hormones may play a role in the subsequent development of colorectal cancer has accumulated from time trends in colorectal cancer rates and from epidemiologic studies. Using data available from the Cross Cancer Institute Northern Alberta Cancer Program, the relationship of parity, exogenous hormones and colorectal cancer was evaluated. Five hundred and twenty-eight colon cancer and 192 rectal cancer cases were identified and 349 nonendocrine cancers were selected as controls. All subjects were diagnosed and interviewed between 1969 and 1973. Protective associations between previous pregnancies and colorectal cancer were found in women over age 50 at diagnosis (OR = 0.5, 95% CI = 0.3-0.9) and in women who never used exogenous hormones (OR = 0.3, 95% CI = 0.2-0.7). Results from this study provide additional support for earlier suggestions that parity may have a protective effect against the development of colorectal cancer, similar to the effects reported in the case of breast, endometrial and ovarian tumors.