Objectives: Several studies have found an inverse association between older age at last birth and endometrial cancer risk. A nested case-control study was undertaken to examine the influence of this and other aspects of reproductive patterns on the risk of developing endometrial cancer.
Methods: Among women born in 1925 and later, 4,839 eligible patients were identified in the Swedish Cancer Register. For each case, five individually age-matched controls were randomly selected from a population-based Fertility Register. Relative risks were estimated from odds ratios obtained from conditional logistic regression analyses.
Results: Compared to uniparous women, childless women were at a higher risk of endometrial cancer (odds ratio [OR] = 1.38, 95% confidence interval [CI] = 1.25-1.52). This association was stronger in younger (< 50 years) than in older (50+ years) women. At all ages of first birth, a delivery was associated with a reduced risk of endometrial cancer that slowly diminished with time. Among parous women, the risk decreased by almost 20% for each additional live birth (OR = 0.81, CI = 0.78-0.84). In an analysis limited to women with two or more births that compared the independent effects of age at first and at last birth, only older age at last birth was associated with a lowered risk of endometrial cancer. The risk decreased at a rate of about 15% per five-year delay of last birth.
Conclusions: Endometrial cancer is often referred to as the prototype hormonally-determined disease in women. However, our findings give further support to the hypothesis that a birth may not only affect risk through hormonal influences, but possibly also through mechanical shedding of cells that have undergone malignant transformation.