Purpose: It has long been held that parity reduces risk of breast cancer. However, accumulating evidence indicates that the effects of parity, as well as breastfeeding, may vary according to estrogen receptor (ER) status. We evaluated these associations in a case-control study among African-American women in New York City and New Jersey.
Methods: In the Women's Circle of Health Study, including 786 African-American women with breast cancer and 1,015 controls, data on reproductive histories were collected from in-person interviews, with tumor characteristics abstracted from pathology reports. We calculated number of live births and months breastfeeding for each child, and examined each in relation to breast cancer by ER status, and for triple-negative (TN) breast cancer.
Results: Although associations were not statistically significant, having children was associated with reduced risk of ER+ breast cancer [odds ratio (OR) 0.82, 95 % confidence interval (CI) 0.58-1.16], but increased risk of ER- tumors, with associations most pronounced for TN breast cancer (OR 1.81, 95 % CI 0.93-3.51). Breastfeeding gave no additional benefit for ER+ cancer, but reduced the risk of ER- disease associated with parity.
Conclusions: Accumulating data from a number of studies, as well as our own in African-American women, indicate that the effects of parity and breastfeeding differ by ER status. African-American women are more likely to have children and not to breastfeed, and to have ER- and TN breast cancer. It is possible that breastfeeding in this population could reduce risk of more aggressive breast cancers.