Menstrual and reproductive factors and endometrial cancer risk: Results from a population-based case-control study in urban Shanghai.

The purpose of our study was to evaluate the association of menstrual and reproductive factors with the risk of endometrial cancer. In a population-based case-control study conducted in urban Shanghai, in-person interviews were completed for 833 women aged 30-69 years and an equal number of controls frequency-matched to cases by age. All cases were newly diagnosed with endometrial cancer between January 1, 1997 and December 31, 2001. The unconditional logistic regression model was employed to derive the adjusted odds ratios (ORs) of endometrial cancer and 95% confidence intervals (CIs) in relation to menstrual and reproductive factors. Earlier menarche age, particularly among premenopausal women, and later menopausal age were associated with an elevated risk of endometrial cancer. A clear dose-response relation between endometrial cancer risk and years of menstruation was observed (p for trend < 0.01). Compared to women ever having a pregnancy and women ever having had a live birth, respectively, nulligravity and nulliparity were both associated with a more than one-fold elevated risk of endometrial cancer. Both completed (OR = 3.02, 95% CI 1.10-8.32 for women never having a complete pregnancy) and incomplete pregnancy (OR = 0.69, 95%CI 0.55-0.87) conferred a protective effect against endometrial cancer, and the protective effect appeared to increase with total number of pregnancies (p for trend = 0.01). The effect of pregnancy on endometrial cancer remained unchanged with increasing time since the last pregnancy. Stillbirth and age at first pregnancy was unrelated to endometrial cancer risk. Our study suggests that prolonged menstruation was related to an increased risk of endometrial cancer while pregnancy, including induced abortion, reduced the risk of endometrial cancer.