TRANSIENT INCREASE IN THE RISK OF BREAST-CANCER AFTER GIVING BIRTH

Background. The effect of pregnancy on the risk of breast cancer is no t clear. We tested the hypothesis that the risk of breast cancer incre ases transiently after pregnancy but then falls to a level below that of age-matched nulliparous women. Methods. We conducted a case-control study of a nationwide cohort in Sweden, using a computerized record l inkage between the Cancer Registry and the Fertility Registry. The stu dy; subjects were women born from 1925 through 1960 who were resident citizens of Sweden at the time of the 1960 census. A total of 12,666 p atients with breast cancer were compared with 62,121 age-matched contr ol subjects. We used conditional logistic regression to estimate odds ratios for the development of breast cancer at different ages, accordi ng to maternal age at first delivery (in uniparous as compared with nu lliparous women) and age at second delivery (in biparous as compared w ith uniparous women). Results. Uniparous women were at higher risk of breast cancel than nulliparous women for up to 15 years after childbir th and at lower risk thereafter. The excess risk was most pronounced a mong women who were older at the time of their first delivery (odds ra tio 5 years after delivery among women 35 years old at first delivery, 1.26; 95 percent confidence interval, 1.10 to 1.44). Women who had tw o pregnancies had a less striking increase in risk. Conclusions. Pregn ancy has a dual effect on the risk of breast cancer: it transiently in creases the risk after childbirth but reduces the risk in later years. In women with two pregnancies, the short-term adverse effect is maske d by the long-term protection imparted by the first pregnancy. A plaus ible biologic interpretation is that pregnancy increases the short-ter m risk of breast dancer by stimulating the growth of cells that have u ndergone the early stages of malignant transformation but that it conf ers longterm protection by inducing the differentiation of normal mamm ary stem cells that have the potential for neoplastic change.