Dense breasts have more glandular and connective tissue and less fat, while nondense breasts are mostly fat. Like tumors, dense tissue looks white on a mammogram and is thus more difficult to evaluate, whereas fat cells provide clearer contrasts.
One reason why breast density increases cancer risk is that very high density may indicate increased breast tissue growth, which may predispose to developing cancer. Moreover, tumors are more likely to go undetected on mammograms of dense breasts. That means that women with dense breasts face a double whammy: They are at increased risk for breast cancer and the cancer may be missed by mammography.
An estimated 30 to 40 percent of U.S. women have dense breasts. They are more common in younger women, those who have never had children or had them when older and women who have been on hormone therapy. Heavier women tend to have less dense breasts, because their breasts tend to be larger and fattier (however, obesity increases the risk of postmenopausal breast cancer for other reasons). Increased breast density is also associated with a family history of breast cancer. Density tends to decrease with every pregnancy and with advancing age after menopause as glandular tissue in the breast decreases and fat increases— but it’s not clear if this lowers breast cancer risk.
Breast density also varies among different ethnic groups, some research suggests. In one study, South Asians and Afro-Caribbean women living in the U.K. had lower density than Caucasians, which may help explain the differences in breast cancer risk seen in these populations. Body weight, hormone use and reproductive history accounted for some—but not all—of the ethnic differences in density, the researchers found. Studies of twins add to the evidence that breast density has a genetic component.