Risk factors for breast cancer in younger women.

Epidemiologic studies of breast cancer that have included younger, premenopausal women in their populations have found that factors that predict altered risk of breast cancer after menopause can have different or even reversed effects before menopause. We have reviewed the literature on risk factors for breast cancer and their associations with breast cancer in general, and compared the risks for younger women with those for women diagnosed at older ages. Race, parity, and large body size are factors that may have opposite effects on breast cancer risk in younger and older women. Other factors of particular significance in the etiology of early-onset breast cancer include a late age at first birth, never having lactated, oral contraceptive use at early ages or of long duration, a family history of breast cancer, and a history of proliferative benign breast disease.