(dailyRx) An aggressive form of breast cancer that isn't fed by estrogen is more common in African American women. New research suggests this type of cancer is seen more frequently among black women who don't breastfeed their babies.

Breast cancers that aren't driven by the hormones estrogen or progesterone are extemely difficult to treat. These are called estrogen receptor negative (ER-) and progesterone receptor negative (PR-) cancers.

These cancers, which don't currently have effective treatments, show up far more often in African American women than in white women. Researchers at Boston University believe the difference may be the result of black women more often choosing not to breastfeed their children.

Breastfeed your children for your own health and the health of your baby.

Researchers found increased risks of ER-/PR- breast cancers among women who had  given birth to two or more children and had not breastfed their infants. These findings come from the Black Women's Health Study, which has been following and surveying 59,000 African American women every other year since 1995.

During the 14-year follow-up, researchers found that women who two or more children were 50 percent more likely to have ER-/PR- cancers. This increased risk, though, was only among women who had not breastfed.

Interestingly, just the opposite is true for ER+/PR+ breast cancers, which are seen more frequently in white women. Giving birth to more than two children results in a decreased risk of this type of cancer. Breastfeeding - which is more popular among white women - doesn't affect these risks.

Lead author, Julie Palmer, ScD, MPH, a senior epidemiologist at the Slone Epidemiology Center and professor of epidemiology at Boston University School of Public Health, says these findings combined with other recent studies, suggest that breastfeeding can help decrease risks of developing the aggressive ER-/PR- breast cancers that are more common for African American women.

This study was published online in Cancer Epidemiology, Biomarkers & Prevention.