Giving birth to two or more children is linked to estrogen and progesterone receptor negative cancers in women who didn't breastfeed, U.S. researchers say. Lead author Julie Palmer, a senior epidemiologist at the Slone Epidemiology Center and a professor of epidemiology at Boston University School of Public Health, said the findings are based on the ongoing Black Women's Health Study, which tracked 59,000 African-American women since 1995. In 14 years of follow-up, 318 women developed breast cancers negative for estrogen and progesterone receptors, while 457 developed breast cancers with positive estrogen and progesterone receptors, Palmer said. The study, published in the journal Cancer Epidemiology, Biomarkers & Prevention, found giving birth to two or more children was associated with a 50 percent increase in the incidence of negative for estrogen and progesterone receptors breast cancer, but the association was not present among women who breastfed. "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," Palmer said. Copyright 2011 United Press International, Inc. (UPI). Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.