"We found that, overall, women who had a history of migraines had a 30 percent lower risk of breast cancer compared to women who did not have a history of such headaches," said Christopher Li, a breast-cancer epidemiologist and associate member of the Hutchinson Center's Public Health Sciences Division in Seattle, Washington.
Li was lead author on the study that appeared in the November issue of Cancer Epidemiology, Biomarkers and Prevention.
Migraine history seemed to curb risk of the most common subtypes of breast cancer: those that are estrogen-receptor or progesterone-receptor positive, the study found.
Those tumors have estrogen or progesterone receptors, or both, on cell surfaces, which makes them more responsive to hormone-blocking drugs than tumors that lack those receptors.
While the biological mechanism behind the association between migraines and breast cancer is not entirely understood Li and colleagues suspect that it has to do with hormone fluctuations.
"Migraines seem to have a hormonal component in that they occur more frequently in women than in men, and some of their known triggers are associated with hormones," Li said.
"For example, women who take oral contraceptives 'three weeks of active pills and one week of inactive pills to trigger menstruation' tend to suffer more migraines during their hormone-free week," he said. Conversely, pregnancy -- a high-estrogen state -- is linked to a significant decrease in migraines.
"By the third trimester of pregnancy, 80 percent of migraine sufferers do not have these episodes," he said.
Estrogen is known to stimulate the growth of hormonally sensitive breast cancer.
According to the US National Cancer Institute, if current rates do not change, one in eight women will develop breast cancer over the course of her lifetime.