''For women with a family history of breast cancer, this suggests an extra benefit [of breastfeeding] is, it may reduce the risk of breast cancer," says Alison Stuebe, MD, an assistant professor of obstetrics and gynecology at the University of North Carolina at Chapel Hill, the lead author of the study. It is published in the Archives of Internal Medicine.
While previous studies have also suggested a link between breastfeeding and reduced breast cancer risk, results have been mixed, Stuebe writes. Studies in which women who already have breast cancer are asked about their breastfeeding history can be flawed by "recall bias," she says.
''Our goal was to collect information before the diagnosis and follow women," Stuebe tells WebMD.
Stuebe and her colleagues drew information from 60,075 women who were participants in the Nurses' Health Study II from 1997 to 2005 and had given birth.
The women answered questions about demographics, body measurements, and lifestyle factors every two years, and described their breastfeeding practices. They were asked about family history of breast cancer and if they had been diagnosed with invasive breast cancer.
By the end of the follow-up in June 2005, Stuebe's team found 608 cases of premenopausal invasive breast cancer, with 99% of the cases verified by medical records. The woman's average age at diagnosis was 46.
''Overall, in the whole group of women we studied, women who had breastfed were 25% less likely to develop premenopausal breast cancer than women who had never breastfed," says Stuebe, who conducted the research while at Brigham and Women's Hospital and Harvard Medical School in Boston.
Family History of Breast Cancer
When the researchers looked separately at the women without a family history and those with a family history of breast cancer (mother, sister, or grandmother), they found ''almost the entire effect could be accounted for by women with a family history," she tells WebMD.
Among those with a family history, those who had breastfed had a 59% reduced risk for premenopausal breast cancer compared to those who never breastfed. The breastfeeding did not have to be exclusive breastfeeding, without formula use.
To understand better the difference between the overall risk reduction and the reduction in those with a family history, Stuebe offers this analogy: Suppose the Los Angeles Lakers and a group of 5-year-olds had a free-throw contest. Overall, the group may have made, say, 60% of the free throws. But when you look separately at the successful free throws made by the basketball stars vs. those made by the kids, the results will undoubtedly be driven entirely by the Lakers.
The risk reduction for women with a family history of breast cancer who breastfeed, Stuebe says, is comparable to that found in high-risk women who take hormonal treatments such as tamoxifen.
''For women without a family history," she tells WebMD, ''it may be that their rates of breast cancer are so low we don't detect a difference or there may not be a protective association."
The protective effect began with three months of breastfeeding, she tells WebMD. That's three months total, she says, not just for a single child. So a mother may have breastfed two children for a month and a half each and gotten the benefit, for instance.
''It is a huge reduction in risk," says Amanda Phipps, a pre-doctoral research associate at the Fred Hutchinson Cancer Research Center in Seattle, of the nearly 60% decreased risk in women who breastfeed and have a family history of breast cancer.
''I find it very interesting," says Phipps, who has researched the link, too. "But I think because it is a rather novel finding it would need to be replicated in the literature."
In a study published in Cancer last year, Phipps and her colleagues found that certain breast cancer types may be rarer among women who breastfeed their babies for at least six months.
The biology to explain the link is not yet clear, Phipps says.
Even so, she calls the association "exciting" because breastfeeding is an action women can take to reduce their breast cancer risk, while many other risk factors -- such as having a family history -- are not modifiable.