Who Is at Risk? Women are confused about factors that can influence risk of developing breast cancer. Many worry unduly because there is breast cancer in their families. It is only cancer in first-degree relatives -- a mother, sister or daughter -- that might raise a woman's risk above that of the general population. Only 10 percent to 15 percent of breast cancers are familial, and not all of those are hereditary. Environmental factors might also play a role. Now that researchers know which genes are responsible for hereditary breast cancer, a woman who can afford genetic testing can find out if she is at increased risk.

As for 85 percent of cases without a family history, there are several well-established risk factors, and some are amenable to adjustment. The primary risk factor is aging, something we are all stuck with. The risk also increases if menarche comes at early age or menopause starts late -- both lengthen the exposure of breast tissue to high doses of growth-stimulating estrogens. A woman who started to menstruate before age 14 has risk that is 30 percent higher than a woman who reached menarche at 16. Because of improved nutrition, better control of childhood infections and reduced physical activity among girls, the average age of menarche has dropped to less than 13 from 16 in the last 130 years. Likewise, a woman who enters menopause at age 55 or later has risk 50 percent higher than that for a woman whose menopause begins earlier.

Another risk factor is having a first baby late in life or having no biological children, which one study suggests could account for almost 30 percent of the breast cancer cases in this country. A pregnancy carried to term changes breast cells in a way that helps block abnormal growth later. Having first child at age 30 or later, or having no children, nearly doubles the risk of breast cancer, compared with risk faced by a woman who bears her first child before she is 20. Furthermore, the earlier a woman has a child, the more children she is likely to have, and these additional pregnancies further protect her breasts, as does prolonged breast feeding.