Premenopausal women, especially women who have never been pregnant, may have dense breast tissue. These are normal dense breasts. Because fatty breasts are easier for a mammogram to see through, sometimes dense breast tissue can hide boundaries of areas that require more study. If a particular industry needs a better image, an ultrasound is usually the next step.
Dr. Dwight Kaufman. Diagnosis: Mammography: normal dense National Cancer Institute. February 1994.If (zSbL