All Breasts Are Not the Same.
The breast consists of fat and breast tissue as well as connective tissue that holds everything together. Breast density is a measure used to describe the proportion of these different tissues. It is not something that can be determined by breast exam but only by mammogram.
High breast density means that there is increased breast and connective tissue compared to fat. Low breast density means that there is increased fat compared with breast and connective tissue.
Mammographic sensitivity for detecting breast cancer ranges from 90-95 percent in the low-density (fatty) category of breast tissue. Conversely, in women at the other end of the spectrum with extremely dense breast tissue, the sensitivity of mammography is only 40-50 percent.