40% of American women have dense breasts, which are characterized by having more breast tissue than fatty tissue, the two components that make up the breast.

On a mammogram dense tissue looks white. Since masses or lumps also appear white on a mammogram, a suspicious lump may be masked by the dense breast tissue.

Dense breast tissue is also linked with an increase in the risk of developing breast cancer, and women with extremely dense breast tissue have a 4 to 6 times greater risk of developing breast cancer than women without it.

What Causes Dense Breast Tissue?

It’s not clear why some women have a lot of dense breast tissue and others do not. You may be more likely to have dense breasts if you:

  • Are younger. Your breast tissue tends to become less dense as you age.
  • Have a lower body mass index. Women with less body fat are more likely to have more dense breast tissue compared with women who are obese.
  • Take hormone therapy for menopause. Women who take combination hormone therapy to relieve signs and symptoms of menopause are more likely to have dense breasts.