Is the risk of cancer of the corpus uteri reduced by a recent pregnancy? A prospective study of 765,756 Norwegian women.

We have examined the relations between the incidence of cancer of the corpus uteri and pregnancies in a cohort of 765,756 Norwegian women, contributing a total of 9,307,118 person-years in the age interval 30-56 years. Incidence rate ratios (IRR) were calculated by Poisson regression analyses of person-years at risk. Separate analyses were carried out for the 2 main histological subtypes, endometrial carcinomas (554 cases) and sarcomas (112 cases). We observed a decrease in risk of endometrial carcinoma with an increasing number of full-term pregnancies (p < 0.001). The reduction in risk associated with the first pregnancy was more pronounced than that observed for any subsequent pregnancy. The risk of endometrial carcinoma increased with increasing time since last birth (IRR = 1.20, 95% CI = 1.08-1.34 per 5-year time interval). The reduction in risk among parous women compared to nulliparous women diminished with increasing time since last birth. For endometrial carcinoma, the decrease in risk with increasing age at first and last birth disappeared after adjustment for time since last birth. For sarcomas, however, the relation with age at births remained in analyses adjusted for time since birth, and time since birth seemed to be of minor importance as an independent risk factor. Our results support the hypothesis that the reduction in risk of endometrial carcinoma associated with a pregnancy is related to a mechanical shed of malignant or pre-malignant cells at each delivery.