The anatomy of the adult female breast consists of 12 - 20 conical lobes. The base of a lobe lies on top of the pectoral muscles and ribs, and its apex is at the areola and nipple. Lobular (glandular) and ductal tissue lie within each lobe supported by intralobular connective tissue and adipose tissue. There is also extralobular connective tissue which binds the lobes together as well as extralobular adipose tissue.

Adipose tissue is radiolucent, and the radiographically visible densities on a mammogram are the images of lobular elements, ducts, and fibrous connective tissue. Ducts may be seen as thin linear structures emanating from the nipple. Lobules and their ducts are often superimposed with connective tissue structures. Whether the mammographic appearance of a breast will appear more or less radiolucent will depend for the most part on the quantity of extralobular connective tissue.

Unfortunately, there is no "normal" appearance on a mammogram that can be memorized. What constitutes "normal" varies within a wide spectrum. In addition, the appearance of the breast differs during pregnancy and in the postpartum period. This spectrum is due to the differences in breast composition. A breast with a high composition of adipose tissue will appear darker on a mammogram than a breast with a high composition of connective tissue stroma (lighter). From a population perspective, the mammogram will appear radiographically denser in a higher percentage of younger women than in older women. Even for an individual female, the image of breast density may vary over the years. Such changes are often gradual and the trend is generally towards a less dense (higher percentage of fat) breast tissue, but the reverse trend may happen as in the case of weight loss or in the case of hormone replacement therapy.

Because the mammogram is a projected image of superimposed breast structures, it is generally more difficult to detect cancer in a breast with a dense (i.e. lighter in appearance on a mammogram) pattern. Surrounding breast structures may mask a lesion unless there is some distortion of the architecture or there is deposition of calcium. A dense breast pattern, however, has not been proven to be a greater risk for breast cancer. The American College of Radiology (ACR) Breast Imaging Reporting and Data System (BIRADS) has categorized overall breast density into four categories from the "almost entirely fat" breast (category 1) to the extremely dense breast (category 4). See images above. The significance of this classification is to alert the clinician that in the radiographically dense breast, the ability of mammography to detect small cancers is reduced.

Other normal variations of breast tissue include asymmetric patterns and asymmetric size. Although the breasts usually develop symmetrically, differences in the symmetry of breast tissue patterns or breast size are not necessarily abnormal. Without other indices of an abnormal process, such asymmetry may simply be a developmental phenomenon.

In conclusion, there is no one breast pattern that may be classified as "normal." Only from experience can one appreciate the variations of a normal-looking mammogram. While the uncertainty in determining whether a breast appearance falls outside of an acceptable normal range is a concern for interpreters of mammograms, familiarity with the spectrum of a "normal" appearance is to a certain extent essential to detecting abnormalities.