Only a very small percentage of breast cancer cases - maybe 5 to 10 percent - are believed to be hereditary.
Research shows women can reduce their risk of getting breast cancer by doing three things: reducing weight if you're overweight or obese, increasing physical activity and using alcohol in moderation only.
Anne McTiernan, a researcher at Seattle's Fred Hutchinson Cancer Research Center, says you don't have to be model thin. If you're overweight, try to lose about 10 percent in the next year. And you don't need to be a super athlete.
"We're talking about moderate-intensity exercise, something like taking a brisk walk preferable most days of the week for at least a half an hour," she said.
But you only get the benefit of exercise if you lose weight or maintain a normal weight, she said.
For alcohol, no more than about one drink a day, on average. More than that and your risk of breast cancer goes up, not matter what you age.
Exercise, weight control reduce risk of breast cancer
American Cancer Society: Breast Cancer