Incidence of cancer of the large bowel in women in relation to reproductive and hormonal factors.

Female resident of King and Pierce Counties (Washington) in whom carcinoma of the colon or rectum was diagnosed during a 15-month period in 1976-77 were interviewed regarding their menstrual and reproductive histories. A random sample of women from the same population was interviewed for comparison. On the average, women with colon cancer had given birth to fewer children than had controls; compared to the incidence of colon cancer in nulliparous women, the incidence in women with 1 or 2 children was reduced by 30%, whereas the incidence in women with 3 or more children was reduced by 50% (P=0.004). No association was present between parity and rectal cancer. Neither contraceptive nor noncontraceptive estrogen use was related to the incidence of colon cancer. Use of oral contraceptives was more common among women with rectal cancer than among controls, but this result could well have arisen by chance (P=0.09). The data were not adequate to determine whether it was the inability to conceive and deliver a child that was related to colon cancer or the failure to undergo the physiologic changes that accompany pregnancy. Nonetheless, the association of low parity with the incidence of colon cancer has now observed several times, in each instance to a moderately strong degree. This observation suggests that events of reproductive life have a bearing on a woman's subsequent risk of developing colon cancer.