Breast Density

What is breast density?

A woman’s breast is primarily made up of fat and fibroglandular breast tissue, which is the network of milk glands and ducts designed to produce and transport milk to the nipple for breast feeding. Fibrous tissue, also known as connective tissue, supports the breast and holds everything in place. Dense tissue is comprised of fibrous connective tissue and glandular tissue.


Breast density is the ratio of volume of dense tissue to the total volume of the breast:

  • High breast density means there is a greater volume of fibroglandular breast tissue compared to fat.
  • Low breast density means there is a greater volume of fat compared to fibroglandular breast tissue.

All women have varying volumes of fatty and fibroglandular breast tissues in their breasts, and that will change how the mammogram looks, from person to person. A typical distribution of breast density in Western women is shown below.

BI RADS distribution3

Breast density changes throughout a woman’s life and in response to a number of factors. It decreases over time due to involution (where glandular tissue is replaced with fat)—the video below shows how breast density changes for a woman over a period of 8 years and on different x-ray systems (i.e. GE Healthcare, Siemens, Hologic).

GIF bg0003

Genetics, body mass index, monthly hormonal cycles, age at first childbirth, and use of post-menopausal hormone replacement can also change breast density.