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.
Factors Associated With Increased Risk
Reproductive factors resulting in increased duration of exposure to endogenous estrogen, such as early menarche, nulliparity, and late menopause, are associated with an increased risk of endometrial cancer. Other factors associated with increased risk, such as obesity and polycystic ovary syndrome, may also be related to increased estrogen exposure.
The first prospective investigation of endogenous estrogens and the risk of endometrial cancer was a case-control study nested within the New York University Women's Health Study. Results suggest an increased risk of endometrial cancer associated with postmenopausal levels of endogenous hormones including estradiol, percent-free estradiol, and estrone. Conversely, risk was decreased with higher levels of percent sex hormone-binding globulin (SHBG)-bound estradiol and SHBG. Analyses conducted prior to adjustment for hormone levels indicated a positive association with body mass index (BMI). After adjustment for estrone level, the positive association of BMI with risk of endometrial cancer was attenuated, suggesting that hormone levels may be an intermediate effect of body weight.