What Breast Density Says About Your Future Breast Health

Do you know your breast density? Only 39 percent of women say their healthcare provider has discussed their breast density with them, and more than 30 percent say they don't know anything about it.

The fact is breast density has serious implications for breast health and screening. Here's the scoop.

Defining breast density

The fatty tissue gives a breast its size and shape. Breast density refers to the amount of fibrous and glandular tissue: Glandular tissue comprises the areas that make milk (lobes) and the tubes (ducts) that transfer milk to the nipple, and fibrous tissue holds breast tissue in place. Breasts with high proportions of fibrous and glandular tissue are considered denser, and those with a higher percentage of fatty tissue as less dense.

The four categories of breast density are assigned letters, A through D (not to be confused with cup size). About 10 percent of women have breasts composed entirely of fatty tissue (A); 40 percent have primarily fatty breasts with some dense tissue throughout (B). Another 40 percent of women have breasts that are evenly dense throughout (C), and 10 percent of women have extremely dense breasts (D).

The first two categories—A and B, or 50 percent of all women—are considered to have low-density breasts. Conversely, women in the second two groups—C and D, the other 50 percent—are considered to have high-density breasts.

Breasts with high proportions of fibrous and glandular tissue are considered denser, and those with a higher percentage of fatty tissue as less dense.

High breast density is more likely in women with a family history of it, as well as women who are younger, pregnant or breastfeeding, who have lower body weight, or are taking hormone replacement therapy. Older women, women with a family history of less dense breasts, women with children, and those taking a medication called tamoxifen—a hormonal therapy used in the treatment of breast cancer that blocks the effects of estrogen in the breast tissue—are more likely to have low-density breasts.

Breast density and breast cancer

Breast density plays a role in cancer risk in multiple ways. First, women with denser breasts have a higher risk of developing breast cancer, the reasons for which remain unclear. That said, women with high-density breasts are not more likely to die of breast cancer.

Having dense breasts makes cancer diagnosis problematic as the higher density makes it more difficult to spot a tumor on a mammogram, resulting in missed or late diagnoses, particularly for small tumors. Dense breasts can also hide other abnormalities, like calcifications. Because of this, women with denser breasts are often called back for a follow-up after a mammogram, perhaps to have additional tests such as a breast ultrasound, MRI (magnetic resonance imaging) or tomosynthesis (like a 3D form of a mammogram), which are more likely to detect breast cancer in dense tissue.

Women with dense breasts are also more likely to get diagnosed with what's called interval breast cancer—diagnosed within 12 months of a normal mammogram.

Understanding your breast density

Breast density is determined solely by a mammogram—you can't tell by a manual exam or by simply looking at your breasts. Nor is breast density correlated with breast shape or size.

Some states require that a woman's breast density be disclosed to her with her mammogram results, which assigns her breasts to one of the four categories of breast density. If your category isn't explained to you when you get your mammogram results, ask about it. If you have dense breasts, talk to your doctor about your breast cancer risk. Keep in mind, though, that breast cancer risk is based on a number of factors, including your family history of the disease, age, weight, exercise, substance abuse, personal history and any possible radiation exposure.

Having dense breasts makes cancer diagnosis problematic as the higher density makes it more difficult to spot a tumor on a mammogram, resulting in missed or late diagnoses.

A number of states now require that women with dense breasts have supplemental health coverage for additional breast imaging. These tests may not be covered by insurance, however, and they're more likely to give a false positive, indicating you have cancer when you don't.

Strategize with your doctor about managing your overall breast cancer risk and your protective factors. Some determinations in your lifestyle—exercise, diet, body weight and alcohol consumption—are modifiable. Decreasing your risk factors and increasing your protective factors will give you the best shot at preventing breast cancer.