M. Lambe et al., MATERNAL RISK OF BREAST-CANCER FOLLOWING MULTIPLE BIRTHS - A NATIONWIDE STUDY IN SWEDEN, CCC. Cancer causes & control, 7(5), 1996, pp. 533-538
The association between multiple births and subsequent maternal breast
cancer risk was explored in a nested case-control study in Sweden enc
ompassing 19,368 parous women with breast cancer diagnosed up to age 6
5 years, and 100,459 parous controls. Among cases and controls, there
were 329 and 2,031 women, respectively, with a history of at least one
live multiple birth. Compared with singleton mothers, breast cancer r
isk was 12 percent lower (odds ratio = 0.88, 95 percent confidence int
erval = 0.78 - 0.99) in women who had had a multiple birth. After stra
tification for age at diagnosis, evidence of a significant inverse ass
ociation was found only in women aged 54 years or younger. Birth order
of the multiple pregnancy had no apparent risk-modifying effect. Age
at earliest multiple birth was unrelated to breast cancer risk. The in
verse association between twinning and breast cancer risk may reflect
protective physiological features of twin pregnancies. Further researc
h is needed to investigate the role, if any, of increased levels of st
eroid hormone-binding globulins in mothers of twins and the proposed i
nhibitory effects of human chorionic gonadotropin and alpha-fetoprotei
n, both of which are increased during multiple gestations, on breast c
arcinogenesis, Breast feeding patterns in mothers of twins also may mo
dify their risk of developing breast cancer.