Age at first pregnancy and risk of colorectal cancer: a case-control study.

Results from a case-control study of colorectal cancer conducted in Toronto and Calgary, Canada, are reported with respect to pregnancy-related variables. A total of 158 female "ever-married" colon cancer cases, 71 rectum cancer cases, 242 neighborhood controls, and 257 hospital controls were interviewed to obtain a complete pregnancy history. The results indicate a strong protective effect of early age at first pregnancy for both colon and rectum cancers, with little or no effect noted for the total number of pregnancies. There is a suggestion that age at first pregnancy may have a greater effect on right colon cancer than on left colon cancer. In addition, there also is a suggestion that non-live-birth outcomes may be a risk factor for both colon and rectum cancer. The similarity of these results to those of other studies on large bowel cancer and on breast cancer support the hypothesis that carcinogenesis in the large bowel may be at least partly mediated by female sex hormones.