The four-year study included more than 1,000 Mexican and Mexican-American breast cancer patients, and concluded that Mexican women who breast-feed and have more children may be at increased risk for aggressive breast cancer.
"We found that breast-feeding in women of Mexican descent is associated with triple-negative breast cancer," said study author Maria Elena Martinez, a cancer researcher at the University of California, San Diego. "This was quite surprising. No other study has seen this correlation before. Most studies show health benefits of breast-feeding."
The study only found an association between breast-feeding, multiple childbirths and breast cancer risk, however. It did not prove a cause-and-effect link.
Triple-negative breast cancer is an aggressive form of breast cancer that's difficult to treat.
Specifically, the researchers found that women of Mexican descent who breast-fed for 12 months or longer had more than twice the increased risk of triple-negative breast cancer. They were younger at diagnosis and younger during their first full-term pregnancy.
Patients who had three or more children were also more likely to have triple-negative breast cancer, according to the study recently published online in the journal Cancer Epidemiology, Biomarkers & Prevention.
"Our results are both puzzling and disconcerting because we do not want to give the wrong message about breast-feeding," Martinez said in a university news release. "If you treat breast cancer as one disease, breast-feeding is beneficial to both mother and baby. That should not be dismissed."
She said further research is needed.
The U.S. Office on Women's Health has more about breast-feeding.