Women who breastfeed are 20 percent less likely to develop a particularly aggressive form of breast cancer, a large international study finds.

Hormone-receptor negative is a particularly aggressive type of breast cancer. Women with HRN breast cancer comprise 20 percent of all breast cancer cases, and this form of the disease is also more commonly diagnosed in women under age 50. The subtype known as triple negative breast cancer (TN) also falls into this group.

HRN and TN breast cancer are more often deadly because they tend to be diagnosed at later stages, respond to fewer treatment options, and are less likely to be cured by current therapies.

In the absence of the receptors for estrogen, progesterone, and HER2, medicines that target these receptors – such as tamoxifen, aromatase inhibitors, Herceptin, and Perjeta – are ineffective and thus have no role in treating these patients.

Women of African American or Sub-Saharan African descent are more likely to be diagnosed with HRN breast cancers, as are women with the BRCA1 gene mutation.

Other factors may put these women at even higher risk for developing HRN breast cancer, including obesity and multiple early pregnancies. Furthermore, women with these multiple risk factors are least likely to breastfeed.

The researchers said the study, which was published in the journal Annals of Oncology, underscored the importance of having healthcare professionals reinforce this message.