Epidemiological studies of in situ breast cancer are sparse, and the r
ole of reproductive history, an established risk modifier for invasive
breast cancer, remains incompletely investigated. To examine possible
associations with parity and age at first birth, we undertook a case-
control study nested in a nationwide cohort of Swedish women. The repr
oductive history of 1,368 women aged 65 or younger with a diagnosis of
carcinoma in site of the breast were compared with that of 6,837 age-
matched controls drawn randomly from a population-based Fertility Regi
stry. Statistical analyses were performed by conditional logistic regr
ession. Compared to nulliparous women, ever-parous women were at a red
uced risk of carcinoma in situ of the breast. The risk decreased with
number of live births, with the estimated risk reduction in the highes
t parity group (5 +), being of the same magnitude as that reported for
invasive breast cancer. By contrast, a positive association with incr
easing age at first birth was somewhat less pronounced than that obser
ved previously in the same data set with respect to invasive breast ca
ncer. Our findings indicate that parity affects the risk of invasive b
reast cancer and carcinoma in situ similarly, whereas the effect of ag
e at first birth appears to be weaker for the risk of carcinoma (C) 19
98 Wiley-Liss, Inc.