Originally published by 2 Minute Medicine® (view original article). Reused on AccessMedicine with permission.

Full-term pregnancies are strongly associated with a reduced risk of endometrial cancer. Whether this protective association is driven by fecundity, a specific process that occurs during pregnancy, or cumulative number of months pregnancy, however, is unknown. In this cohort study, 2,311,332 Danish women born between 1935 and 2002 were followed for an average of 24.8 years to explore the association between pregnancy duration and risk of endometrial cancer. These women had 3,947,650 pregnancies, of which 671,560 were induced abortions (median gestational age 8 weeks, IQR 7-9 weeks), and 3,276,090 were births (median gestational age 40 weeks, IQR 39-41 weeks). During study follow-up, 6,743 women developed endometrial cancer. Researchers found that both induced abortions and births after a first pregnancy were strongly associated with a reduced risk of endometrial cancer after adjustment for age, time period, and socioeconomic factors (induced abortion: RR 0.53, 95% CI 0.45 to 0.64; childbirth: RR 0.66, 95% CI 0.61 to 0.72). Each subsequent pregnancy was also associated with risk reduction, whether it ended in induced abortion (RR 0.81, 95% CI 0.77 to 0.86) or childbirth (RR 0.86, 95% CI 0.84 to 0.89). There was no difference in the relative risk of endometrial cancer when stratifying by age at first pregnancy (p=0.69), nor time since pregnancy (less than 10 years vs. 10 years or greater, p=0.94). The results were also not affected by obesity, birth cohort, or fecundity. This study was limited by the lack of data regarding lifetime use of oral contraceptives, which has also been associated with a decreased risk of endometrial cancer. Overall, this study indicates that pregnancy is associated with a reduced the risk of endometrial cancer, regardless of whether the pregnancy ends in induced abortion or birth, suggesting that the driving protective factor is either a women’s fecundity or a biological process occurring early in gestation.

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