For women in their forties, having mammograms on a regular basis reduces their chances of dying from breast cancer by 16 percent. For women age 50 to 69, there is strong evidence that screening with mammograms on a regular basis reduces breast cancer deaths by 25 percent to 30 percent. Mammography may not help a woman with a fast-growing or aggressive cancer that has already spread to other parts of her body before being detected. False negatives occur when mammograms appear normal even though breast cancer is actually present. Overall, mammograms will miss approximately 20 percent of the breast cancers that are present at the time of screening.
False negatives occur more often with younger women than with older women.
The dense breasts of younger women contain many glands and ligaments, which make breast cancers more difficult to spot in mammograms. Most abnormalities will turn out not to be cancer. False positives are more common in younger women than older women. About 30 percent of women ages 40 to 49 have a false positive mammogram, as compared with about 25 percent for women age 50 and older. However, all women must undergo followup procedures when they have an abnormal mammogram.