Grand multiparity and the risk of breast cancer: population-based study inFinland

Objectives: The significance of reproductive factors on breast cancer risk has so far been characterized in populations with 5-paras as the highest ca tegory of parity. We extended these studies to a nationwide cohort of women with at least five births (grand multiparas = GM) by assessing the signifi cance of parity, age at first birth, and average birth interval to the risk of breast cancer. Methods: The study cohort obtained from the Population Register of Finland comprised 86,978 GM-women; the incidence of cancer cases was obtained from the populated-based Finnish Cancer Registry. During a follow-up of about 2 million person-years, 1508 breast cancers were obtained. Standardized incid ence ratios (SIRs) were calculated by dividing the number of observed cases by the number expected on the basis of national rates. Results: In the GM cohort the incidence of breast cancer was low (SIR 0.55, 95% confidence interval 0.52-0.58). The relative risk decreased significan tly from 5-paras (SIR 0.60, adjusted for the other study variables) to 8-pa ras (SIR 0.40). The increase in the age at first birth from less than 20 ye ars to 30+ years nearly doubled the risk (SIR from 0.40 to 0.73). Parity wa s a significant risk determinant only in ductal cancer, while shortening th e birth interval was protective only in lobular cancer. The incidence of ad vanced breast cancer among GM-women exceeded the population rate in premeno pausal women and in women with first birth at the age of 30 years or more. Conclusions: Our study demonstrated that young age at first birth and incre asing number of births were independent and powerful protective factors fro m the fifth child onwards, while birth interval was weak in this respect. T he tumor morphology and the clinical advancement of malignancy modified the dependence of breast cancer risk on reproductive variables.