And the greater density of their breasts makes it harder to spot tumors. As a result, mammography for them may produce less benefit. And that can make the test's risks-mainly false alarms and the anxiety and biopsies they trigger-more disturbing. On the other hand, breast cancer in younger women tends to be especially aggressive. The more breast-cancer risk factors you have the more sensible testing in your 40s becomes.

But most cancers occur in women with no known risk factors. If you're in that group, the decision boils down to how worried you are about breast cancer and how prepared you are for the possibility of a false-positive result.