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

The significance of reproductive factors on breast cancer risk has so far been characterized in populations with 5-paras as the highest category of parity. We extended these studies to a nationwide cohort of women with at least five births (grand multiparas = GM) by assessing the significance of parity, age at first birth, and average birth interval to the risk of breast cancer.

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 incidence ratios (SIRs) were calculated by dividing the number of observed cases by the number expected on the basis of national rates.

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 significantly from 5-paras (SIR 0.60, adjusted for the other study variables) to 8-paras (SIR 0.40). The increase in the age at first birth from less than 20 years to 30+ years nearly doubled the risk (SIR from 0.40 to 0.73). Parity was a significant risk determinant only in ductal cancer, while shortening the birth interval was protective only in lobular cancer. The incidence of advanced breast cancer among GM-women exceeded the population rate in premenopausal women and in women with first birth at the age of 30 years or more.

Our study demonstrated that young age at first birth and increasing number of births were independent and powerful protective factors from the fifth child onwards, while birth interval was weak in this respect. The tumor morphology and the clinical advancement of malignancy modified the dependence of breast cancer risk on reproductive variables.