A Swedish population-based case-control study confirms these findings. Women who used any type of oral contraceptive had a 30% risk reduction (odds ratio [OR] = 0.7; 95% CI, 0.5-0.9) and women who used progestin-only pills had a 60% risk reduction (OR = 0.4; 95% CI, 0.2-1.4). Women who used COCs for at least 3 years had a 50% risk reduction (OR = 0.5; 95% CI, 0.3-0.7) and those who used COCs for at least 10 years had an 80% risk reduction (OR = 0.2; 95% CI, 0.1-0.4). Overall, risk decreased by 10% per year of COC use and was observed for atypical hyperplasias as well as all grades of invasive tumors.
Several cohort [14,15] and case-control [16,17,18,19,20,21,22,23,24] studies of physical activity and endometrial cancer reveal a weak to moderate inverse relationship, despite varying methods of assessing physical activity levels. For postmenopausal women enrolled in The Netherlands Cohort Study on Diet and Cancer, a 46% reduction (RR = 0.54; 95% CI, 0.34-0.85; P = .002) in risk of endometrial cancer was reported in those women who were physically active 90 minutes or more per day compared with less than 30 minutes each day. One case-control study of 822 endometrial cancer cases and 1,111 population controls showed that regular exercise was associated with a 38% decrease in risk (OR = 0.62; 95% CI, 0.51-0.76) without a trend for increasing duration or intensity of physical activity. The Breast Cancer Detection Project Follow-up Study, using a prospective cohort, did not confirm an association between recent physical activity levels and risk. It is unknown whether physical activity reduces endometrial cancer risk by reducing obesity, by reducing serum estrone levels, or by another mechanism.
Factors Associated With Decreased Risk
Increasing parity and lactation
Decreased risk of endometrial cancer is associated with parity and lactation, perhaps by inhibiting ovulation. A case-control study conducted in Mexico City, among low-risk women, indicates a 58% to 72% reduction in risk of endometrial cancer associated with increasing duration of lactation. A significant trend was seen for duration of lactation and for the number of children breastfed. A population-based case-control study, comparing Wisconsin women who breastfed for at least 2 weeks versus those who did not, was negative (OR = 0.90; 95% CI, 0.72-1.13). Increasing duration of lactation was not associated with a decrease in disease risk, but breastfeeding within the past three decades was associated with reduced risk (OR = 0.58; 95% CI, 0.36-0.96), as was the first breastfeeding after age 30 years (95% CI, 0.28-0.90). The European Prospective Investigation into Cancer and Nutrition observed a decreased risk associated with parity compared with nulliparous women (hazard ratio = 0.65; 95% CI, 0.54-0.77) with a trend of decreasing risk with increasing number of full-term pregnancies (P < .0001). While breastfeeding for more than 18 months was associated with a decreased risk, the association attenuated and was no longer statistically significant after adjusting for the numbers of full-term pregnancies.