Childbearing in marriage and mortality from breast cancer in Norway.

The relationship between childbearing and mortality from breast cancer has been studied in a cohort of 822,593 currently married Norwegian women with information on parity from the Census in 1970 and follow-up till 1985. All age groups of parous women showed significant trends of decreasing mortality rates with increasing parity. Nulliparous women had the same mortality rates as uniparous women in all age groups. In a stratified analysis, for age group 45-74 years, the relative risk for uniparous women was between 3.8 and 4.5 dependent on age at first birth compared to women with 8-9 children, age at first birth before the age of 25 and last birth after the age of 30 years. In a multivariate analysis age and parity were stronger risk factors than age at first birth, while age at last birth was nonsignificant. Among women 45-74 years the population attributable risk of breast cancer mortality due to childbearing was 72%, using women with 8-9 children as the reference group. The use of different definitions of reference group for parity is discussed.