Women who develop breast cancer may have a better chance of survival if they previously breastfed for at least six months, a new study suggests.

Researchers in Norway and Sweden looked at almost 350 women who had undergone surgery for primary breast cancer 20 years before and found that mortality (death) rates were lower among those who had breastfed for six months or more.

The study found that this applied to both breast cancer mortality rates and overall mortality rates.

By assessing the women 20 years after they had been treated for breast cancer, the researchers were able to take into account other causes of death which could have contributed to mortality.

"The clinical conclusion is that women diagnosed with breast cancer, and who have lived long enough to make other causes of death contribute substantially to mortality, have a better survival rate if they have been pregnant and breastfed longer. Our findings require more research on the associations and causal relationships between breastfeeding and all-cause mortality," the researchers concluded.

Details of these findings are published in the journal, Breastfeeding Medicine.