Investigators from the Boston University's Slone Epidemiology Center have reported findings that may shed light on why African American women have a disproportionately higher risk of developing more aggressive and difficult-to-treat breast cancers, specifically estrogen and progesterone receptor negative (ER-/PR-) cancers. The study, which appears online in Cancer Epidemiology, Biomarkers & Prevention, found that high parity（平价，同等） (giving birth to two or more children) was associated with an increased risk of ER-/PR- cancer, but only among women who had not breastfed.

The findings were based on the ongoing Black Women's Health Study, which has followed 59,000 African American women by biennial（两年一次的） questionnaire since 1995.

In 14 years of follow-up, 318 women developed breast cancers negative for estrogen and progesterone receptors (ER-/PR-), while 457 developed breast cancers with estrogen and progesterone（黄体酮） receptors (ER+/PR+). Giving birth to two or more children was associated with a 50 percent increase in the incidence of ER-/PR- breast cancer, but the association was not present among women who had breastfed.

According to the researchers, the results for ER+/PR+ breast cancer, which is more common among white women, were strikingly different. High parity was associated with a decreased risk, and breast feeding had no influence on that association.

"The higher incidence of ER-/PR- breast cancer in African American women may be explained in part by their higher parity and lower prevalence of breastfeeding relative to white women," explained lead author Julie Palmer, ScD, MPH, a senior epidemiologist at the Slone Epidemiology Center and a professor of epidemiology（流行病学） at Boston University School of Public Health.

"Our results, taken together with recent results from studies of triple negative and basal-like breast cancer, suggest that breastfeeding can reduce risk of developing the aggressive, difficult-to-treat breast cancers that disproportionately affect African American women," she said.

This study was supported by a grant from the National Cancer Institute of the National Institutes of Health.