Grand multiparity and incidence of endometrial cancer: a population-based study in Finland

The hormonal background of endometrial cancer is insufficiently characterised. We investigated the significance of parity, age at first birth, intensity between births, length of time from the first to the last birth and length of delivery-free premenopausal period in a cohort of grand multiparous (GM) women, i.e., women with at least 5 births. Data of the Population Register of Finland (86,978 GM-women) and the population-based Finnish Cancer Registry were combined. Standardised incidence ratios (SIRs) were calculated by dividing the number of observed cancer cases by the expected number based on the national incidence rates. Multivariate relative risks (RRs) were estimated by Poisson regression analysis. The SIR for endometrial cancer among GM-women was low [419 cases; SIR=0.57, 95% confidence interval (CI) 0.52-0.63]. The RR of endometrial cancer was 0.58 (95% CI 0.34-0.97) among women giving their first birth at an age of more than 30 years compared to women with first birth before the age of 20. In ages 50+ (94% of endometrial cancer cases), the RR for women with at least 8 births was 0.63 (95% CI 0.44-0.92) compared to those with 5 births, and those with a birth period of 20+ years had RR=0.57 (95% CI 0.34-0.96) compared to those with a period of <10 years, while prolonged average intensity between births showed only a small protective effect. The RR of endometrial cancer also correlated with the length of premenopausal delivery-free period (RR = 0.61, 95% CI 0.44-0.86) for women with a short (<10 years) period compared to women with a long (>15 years) period. Our findings, that a large number of births, old age at first birth, a long birth period and a short premenopausal delivery-free period reduced the risk of postmenopausal endometrial cancer of GM-women, emphasise the protective role of progesterone and the stimulatory role of estradiol in the hormonal background of this disease.