Breast Density

April 10, 2025

This page was reviewed under our medical and editorial policy by Susan Yost, Ph.D., staff scientist, Department of Medical Oncology & Therapeutics Research, City of Hope^® Cancer Center Duarte

Women may feel concerned when they see a breast density finding listed on their imaging report after a mammogram. However, dense breasts are common. Almost half of women aged 40 and older who have a mammogram are found to have dense breasts, according to the National Cancer Institute (NCI).

What Does Dense Breast Tissue Mean?

Dense breast tissue refers to the amounts of glandular, fibrous connective and fatty tissues in a woman’s breasts.

Glandular tissue: This makes up the breast lobules that produce milk and the ducts that carry the milk to the nipple.

Fibrous connective tissue: This holds the breast structure together and is made of strong proteins called collagen, along with fibroblast cells. It is sometimes referred to as dense connective tissue.

Fatty tissue: This fills in the spaces between the fibrous connective tissue and the glandular tissue.

Women who have relatively lower amounts of fatty tissue and relatively higher amounts of glandular and fibrous connective tissues are considered to have dense breast tissue. This dense tissue pattern is more difficult to see through on mammogram images compared to mostly fatty tissue.

Heterogeneously Dense Breasts

There are four levels or categories of breast density, including not dense (almost entirely fatty tissue), scattered areas of fibroglandular density, heterogeneously dense breasts (an even amount of glandular and fibrous connective tissues spread throughout the breasts), and extremely dense (almost entirely glandular and fibrous connective tissues).

When a woman has dense breasts, most of the time she falls into the heterogeneously dense breasts category.

Fibroglandular Breast Density

Glandular and fibrous connective tissues are sometimes referred to as fibroglandular tissue. Fibroglandular breast density is the amount of these two tissues in a breast as compared to fatty tissue.

If a woman’s mammogram report shows scattered fibroglandular densities, this means that fibroglandular tissues are located in different areas of the breast, but not throughout it. This is the second level or category of breast tissue density. It is found in about 40% of women, according to NCI.

What Causes Dense Breast Tissue?

Breast density is related to hormone levels and tends to be higher in women who are:

• Younger in age
• Premenopausal
• Breastfeeding or pregnant
• Weigh less and have a lower body mass index
• Taking hormone replacement therapy

Other things tend to lower breast density. Certain medications, such as the breast cancer treatment drug tamoxifen and aromatase inhibitors (which lower levels of the hormone estrogen in the body), have been shown to decrease breast density levels. Women who have given birth also tend to have lower breast density.

Dense Breast Symptoms

Dense breasts are not considered a disease or abnormal condition, and they do not typically present with symptoms. A woman will not feel breast density using a self-exam of the breast. Instead, the level of density present is found with a mammogram imaging test.

Dense Breasts and Cancer

Some women may wonder what dense breast tissue means, especially in regard to cancer risk. Here is a breakdown of how dense tissue impacts the chances of developing breast cancer.

Cancer Risk

Women with dense breasts have a higher risk of breast cancer compared to women with more fatty breasts, and this risk increases as density increases, according to the U.S. Centers for Disease Control and Prevention. However, breast density does not affect survival rates once breast cancer develops.

Researchers are still exploring why having dense breasts is linked to higher risk of breast cancer. It may be due to the development of more abnormal cells in fibroglandular tissue than in fatty tissue, and efforts are ongoing to determine the exact relationship between cancer and density.

Dense Breast Mammogram Report

Mammogram reports are shared with each woman and her provider. The report includes breast density level. A radiologist, who is a doctor trained to read the results of imaging tests like mammograms, studies the mammography images and determines the density level.

Dense breast tissue shows up as white areas on a mammogram. The radiologist will choose one of four categories of density based on the extent and spread of these white areas.

NCI describes each density category and how many women fall into each category.

Category A (fatty tissue): Few white areas are present, meaning the breasts are mainly fatty tissue. These are not considered dense breasts. About 10% of women have mainly fatty breasts.

Category B (scattered fibroglandular densities): Some scattered white areas are present, although the breasts are mostly of fatty tissues and are not considered dense. About 40% of women have scattered fibroglandular densities.

Category C (heterogeneously dense breasts): More tissue is fibroglandular than fatty, and the fibroglandular tissues are evenly dense throughout. Breast tissue in this category is considered dense and is found in about 40% of women.

Category D (extremely dense): Almost all tissue is fibroglandular, and the breasts are considered extremely dense. About 10% of women have extremely dense breasts.

Dense Breast Mammogram Frequency

For women with dense breasts, it is beneficial to talk to their doctor about the pros and cons of having 3D mammograms (digital tomosynthesis) in place of (or alongside) traditional 2D mammograms. The reason for this is that tumors and lumps show up white on mammograms, just like dense tissue does, so they are hard to detect. With 3D mammography, additional pictures and computer technology are used to make the pictures clearer, which research has shown may help radiologists find more breast cancer.

The U.S. Preventive Services Task Force (USPSTF) does not recommend any additional mammogram screenings for women with dense breasts, but it may be beneficial to discuss a breast cancer screening schedule with the doctor, particularly if a woman has other breast cancer risk factors.

Should Women Worry About Dense Breasts?

Finding out that she has dense breasts should not cause a woman to worry, but should help her understand her risk for developing breast cancer. Doctors use this information to help put together a personalized cancer risk management plan, which may include additional screening tests such as:

• Magnetic resonance imaging (MRI) scans of the breast, which use magnets and computers to take detailed breast tissue pictures
• Ultrasound scans of the breast, which use sound waves to capture images of the breast

Women who are already at high risk for developing breast cancer because of a family history of the disease or other risk factors may especially benefit from these additional screening tests.

The Breast Cancer Program at City of Hope^Ⓡ offers a comprehensive list of services, including assessments to help women understand their risks for developing the disease and advanced screenings to assist in managing health outcomes.