University Of North Carolina researchers reviewed data on about 60-thousand women.
For women who had family members with breast cancer, nursing led to a 59-percent decreased risk for developing pre-menopausal breast cancer themselves that's similar to taking the cancer drug tamoxifen for 5 years.
Researchers say it didn't matter how long women breastfed -- 3 months or 3 years yielded the same results.
Doctors said that protective effect did not apply to women without a family history of breast cancer.