PARITY, AGE AT FIRST AND LAST BIRTH, AND RISK OF BREAST-CANCER - A POPULATION-BASED STUDY IN SWEDEN

Associations between parity and the risk of breast cancer, and the rel ative importance of age at first and age at last birth on breast cance r risk, were estimated in a case-control study nested in a nation-wide cohort of Swedish women born between 1925 and 1960. A total of 12,782 women with breast cancer and five times as many individually age-matc hed controls, aged less than 60 years with concomitant fertility infor mation, were included in the analysis. Increasing parity was associate d with a pronounced decrease in the risk of breast cancer with each ad ditional birth conferring a 10 percent risk reduction (odds ratio 0.90 [95% CI 0.88-0.91]). In an analysis limited to women with two or more parities, and after adjustment for the effects of ages at interim bir ths, the risk of breast cancer increased by about 13 percent for each five year increment in age at first birth (odds ratio 1.13 [1.08-1.19] ). For every five year-increase in age at last birth there was a small risk increase of marginal statistical significance (odds ratio 1.05 [ 1.01-1.09]). The present findings contradict recent claims that age at last birth has a stronger effect than age at first birth on breast ca ncer risk. The dominance of age at first birth as risk modulator is li kely to reflect the protection afforded by the terminal differentiatio n of breast cells induced by a first pregnancy.