Aside from age, "almost all breast cancer is somehow tied to estrogen levels," Saslow says. Gaining a lot of weight as an adult is a risk factor because fat produces estrogen. Getting your first period young and reaching menopause late (after 55) also increase risk because they prolong the body's estrogen exposure. Hormone replacement therapy can increase risk, as can taking birth control pills (slightly), though the effect fades within a few years of going off the Pill. Drinking alcohol increases risk, possibly because it interferes with estrogen metabolism. And giving birth before 30—the younger, the better—offers a protective effect against the disease. After that, childbearing isn't protective and may even increase risk due to changes in the breast tissue that are salutary at an early age but not later.

Some of these factors are controllable (weight, alcohol consumption). Others are not (age). And nobody in her right mind is going to plan a teen pregnancy to reduce breast cancer risk ("Oh, no, all of life should be planned around breast cancer," deadpans Garber). One factor you wouldn't want to change: Being wealthy adds risk. People in richer countries grow up with better nutrition, and girls tend to reach puberty earlier. "If you were to talk to Walter Willett [a Harvard epidemiologist], he calls breast cancer the price of progress," Garber says. (To find out your actual risk, go to

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