Pregnancy Outcomes and Risk of Endometrial Cancer : A Pooled Analysis of Individual Participant Data in the Epidemiology of Endometrial Cancer Consortium

A full-term pregnancy is associated with reduced endometrial cancer risk; however, whether the effect of additional pregnancies is independent of age at last pregnancy is unknown. The associations between other pregnancy-related factors and endometrial cancer risk are less clear. We pooled individual participant data from 11 cohort and 19 case-control studies participating in the Epidemiology of Endometrial Cancer Consortium (E2C2) including 16,986 women with endometrial cancer and 39,538 control women. We used one- and two-stage meta-analytic approaches to estimate pooled odds ratios (OR) for the association between exposures and endometrial cancer risk. Ever having a full-term pregnancy was associated with a 41% reduction in risk of endometrial cancer compared to never having a full-term pregnancy (OR=0.59, 95% confidence interval (CI) 0.56-0.63). The risk reduction appeared greatest for the first full-term pregnancy (OR=0.78, 95%CI 0.72-0.84), with a further ~15% reduction per pregnancy up to eight pregnancies (OR=0.20, 95%CI 0.14-0.28) that was independent of age at last full-term pregnancy. Incomplete pregnancy was also associated with decreased endometrial cancer risk (7-9% reduction per pregnancy). Twin births appeared to have the same effect as singleton pregnancies. Our pooled analysis shows that, while the magnitude of the risk reduction is greater for a full-term pregnancy than an incomplete pregnancy, each additional pregnancy is associated with further reduction in endometrial cancer risk, independent of age at last full-term pregnancy. These results suggest the very high progesterone level in the last trimester of pregnancy is not the sole explanation for the protective effect of pregnancy.