Oral contraceptive use and the risk of endometrial cancer. The Centers for Disease Control Cancer and Steroid Hormone Study.

Use of combination oral contraceptives (OCs) has been reported to decrease a woman's risk of endometrial cancer developing. To investigate this issue further, we analyzed data from a multicenter, population-based, case-control study--the Cancer and Steroid Hormone Study conducted by the Centers for Disease Control, Atlanta. Cases are all women 20 to 54 years old with a first diagnosis of endometrial cancer ascertained through eight population-based cancer registries; controls are women selected at random from the population of these eight areas. Analysis of the first 187 cases and 1,320 controls showed that women who had used combination OCs at some time in their lives had a relative risk of endometrial cancer developing of 0.5 (95% confidence interval, 0.4 to 0.8) compared with never-users. The protective effect occurred in women who had used combination OCs for at least 12 months, and it persisted for at least ten years after the cessation of OC use. The protective effect was most notable for nulliparous women. Nulliparous combination OC users had a risk 0.4 times (95% confidence interval, 0.2 to 0.9) that of nulliparous never-users. These results were not accounted for by differences between cases and controls in health status, parity, infertility, or other potentially confounding variables. We estimate that approximately 2,000 cases of endometrial cancer are averted each year by past and current OC use among women in the United States.