Background There is increasing interest in the role of early life exposures in breast carcinogenesis. We focused on the association of perinatal risk factors with breast cancer risk in a case-control study of women aged 35-80 in Western New York (Western New York Exposures and Breast Cancer Study (the WEB Study), 1996-2001). Methods The study included 845 women diagnosed with primary, incident, histologically confirmed breast cancer, and 1538 controls frequency matched to cases on age, race and county of residence. We conducted extensive in-person interviews regarding perinatal exposures, including birth weight, having been breastfed, birth rank, and other epidemiologic data. Results Birth weight was significantly associated with pre- but not postmenopausal breast cancer risk. With women whose birth weigh was 5.5-7 pounds as reference category, we found an increase in risk associated with birth weigh greater than 8.5 pounds (OR 2.10, 95%CI 1.23-3.58). Risk was also increased for pre-menopausal women who had not been breast fed (OR 1.96, 95%CI 1.28-2.99).Birth order was not associated with breast cancer risk. Conclusions Our findings are consistent with other studies showing increased breast cancer risk associated with birth weight for pre- but not postmenopausal breast cancer. As we had found earlier, having been breastfed was associated with decreased risk. These findings add to the accumulating evidence that early life impacts women’s breast cancer risk.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]