Women who undergo surgery for breast cancer have a better chance of survival if they breastfed for at least six months.

A new study, which was reported in Breastfeeding Medicine, the official journal of the Academy of Breastfeeding Medicine, looked at women 20 years after they underwent breast cancer surgery. It found that not only did breastfeeding lower the risk of dying from breast cancer, it also lowered the risk of dying from all causes.

"This study confirms that the long-term maternal health benefits of breastfeeding are not only preventive in nature, but that it also has the capacity to reduce the severity of breast cancer," says Arthur I. Eidelman, MD, Editor-in-Chief of Breastfeeding Medicine.

A study published last year in the Journal of the National Cancer Institute found that women who had breastfed and developed breast cancer later were 30 percent less likely to see their cancer recur and also 28 percent less likely to die from the disease than woman who didn't breastfeed.

There are a number of possible explanations for why breastfeeding could be associated with a better prognosis once a woman is diagnosed with breast cancer. Women who develop cancer but breastfed in the past tend to have less aggressive cancers than those who didn't breastfeed.

"Women who breastfeed are more likely to get the luminal A subtype of breast cancer, which is less aggressive, and breastfeeding may set up a molecular environment that makes the tumor more responsive to anti-estrogen therapy," said researcher Marilyn Kwan.

According to breastcancer.org, about 1 in 8 women will develop invasive breast cancer during their lifetime. About 246,660 new cases of invasive breast cancer will be diagnosed in 2016 along with about 61,000 additional cases of in situ breast cancer. Over 40,000 American women die from the disease each year.