08/17/2011 08:52 am ET | Updated Oct 16, 2011

Boston University Identifies Contributors To High Breast Cancer Incidence In African-American Women

Researchers at Boston University's Sloan Epidemiology Center have uncovered new evidence that might explain why African-American women have a disproportionately higher risk of developing more aggressive and difficult-to-treat forms of breast cancer.

Their study, based on the ongoing Black Women's Health Study, which has followed 59,000 African American women since 1995, found that giving birth to two or more children was associated with a 50 percent increase in the incidence of estrogen and progesterone receptor negative (ER-/PR-) cancers, but only among women who had not breastfed.

ER- and PR-negative results mean that the growth of the cancer is not supported by the hormones estrogen and progesterone, and therefore does not respond to hormonal therapy (such as tamoxifen or aromatase inhibitors). It's the type of cancer "Good Morning America" anchor Robin Roberts was diagnosed with back in 2007 and accounts for 10 to 20 percent of breast cancers -- more than one out of every 10 -- according to

Over the study's 14 year follow-up, 318 women developed ER-/PR- breast cancers, while 457 developed breast cancers with estrogen and progesterone receptors (ER and PR positive).

What was also striking, was that having two or more children was associated with a decreased risk for ER and PR positive breast cancer, a form that is more common among white women. Breast feeding had no influence on that association.

"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 aggressive, difficult-to-treat breast cancers," explained lead author Julie Palmer, ScD, MPH, a senior epidemiologist at the Slone Epidemiology Center and a professor of epidemiology at Boston University School of Public Health, in a press release.