Objectives: To estimate the magnitude and persistence of the protective effect of use of combined oral contraceptives (COCs) and endometrial cancer risk.
Methods: We performed a nation-wide, population-based case-control study among postmenopausal women aged 50-74 years in Sweden, which included 709 subjects with incident, histopathologically verified endometrial cancer, and 3,368 controls with an intact uterus. We used unconditional logistic regression to calculate odds ratios as estimates of relative risks.
Results: Use of any sort of oral contraceptive decreased risk for endometrial cancer by 30%, while progestin-only pills reduced risk more markedly. For COCs the reduction in risk was noticeable following 3 or more years of use (OR 0.5, 95% CI 0.3-0.7), and increased with duration of intake, reaching 80% lower risk after 10 years of use. The protective effect of COC use was similar for all degrees of tumor differentiation and invasiveness, and remained for at least 20 years after cessation of use. Subsequent use of hormone replacement did not modify these protective effects.
Conclusions: We conclude that COC use confers a long-lasting protection against endometrial cancer risk which is particularly marked for long-term users.