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, c ontributing a total of 9,307,118 person-years in the age interval 30-5 6 years. Incidence rate ratios (IRR) were calculated by Poisson regres sion 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 end ometrial carcinoma with an increasing number of fullterm pregnancies ( p < 0.001). The reduction in risk associated with the first pregnancy was more pronounced than that observed for any subsequent pregnancy. T he 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 car cinoma, the decrease in risk with increasing age at first and last bir th disappeared after adjustment for time since last birth. For sarcoma s, however, the relation with age at births remained in analyses adjus ted for time since birth, and time since birth seemed to be of minor i mportance as an independent risk factor. Our results support the hypot hesis that the reduction in risk of endometrial carcinoma associated w ith a pregnancy is related to a mechanical shed of malignant or pre-ma lignant cells at each delivery. (C) 1995 Wiley-Liss, Inc.