Breast density refers to the amount of fibrous and glandular tissue in a womans breasts compared to the amount of fatty tissue, as seen on a mammogram. Breasts are made up of lobules, ducts, and fatty and fibrous connective tissue. Lobules are the small glands that produce milk, while ducts are the tiny tubes that carry the milk from the lobules to the nipple. Together, the lobules and ducts are referred to as glandular tissue. Fibrous tissue and fat give breasts their size and shape and hold the other structures in place.

Breast density is defined based on the amount of glandular and fibrous tissue as compared to the percentage of fatty tissue. The current mammography classifications split up the density of breasts into four categories:

• Almost entirely fatty (about 10% of women)
• A few areas of dense tissue are scattered through the breasts (about 40% of women)
• The breasts are evenly dense throughout (about 40% of women). This may be called “heterogeneously dense” on the mammography report.
• The breasts are extremely dense (about 10% of women)

Women in the first two categories are said to have low-density, non-dense, or fatty breasts. Women in the second two categories are said to have high-density or dense breasts. About half of women who are 40 years old or older have dense breasts.

Having dense breasts can make it more difficult to interpret mammograms, as dense breast tissue and some abnormal breast changes, such as calcifications and tumors, both appear as white areas in the mammogram, whereas fatty tissue appears as dark areas. Women with dense breasts have a higher chance of getting breast cancer, and the more dense the breasts are, the higher the risk. Scientists don’t know for sure why this is true.