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.