If your mammogram indicate you have dense breasts, you're likely wondering what it means for you.

"Every mammogram report indicates breast tissue density, of which there are four categories — ranging from almost entirely fatty to extremely dense," says Dr. Rajul Mehta, a breast radiologist at Houston Methodist. "Around half of women eligible for mammograms have dense breast tissue."

Where your breasts fall on the density spectrum can be important information because it may affect how your doctor approaches your future breast cancer screenings.

What is dense breast tissue?

We know our breasts contain fatty tissue. It's what gives them their size and shape. But there are two other types of tissue in our breasts — glandular and fibrous.

Glandular tissue is made up of milk-producing glands (called lobules) and the ducts that carry that milk to the nipple. Fibrous tissue helps hold these structures in place.

"Both are more dense than fatty tissue, and together they're called the fibroglandular tissue," says Dr. Mehta. "In women with dense breast tissue, there's more fibroglandular tissue than fatty tissue. And this difference can be seen on a mammogram."

Why do some women have dense breasts and others don't?

The answer isn't entirely clear, but factors that make it more likely include:

• Being younger
• Having less body fat
• Taking hormone replacement therapy after menopause
  
  

"We also see dense breast tissue running in families, so there seems to be a genetic component at play," adds Dr. Mehta.

Does having dense breasts increase breast cancer risk?

"Breast cancer develops in the dense fibroglandular tissue, so the concern with having more of it is that there's an increased number of cells for cancer to develop in," explains Dr. Mehta. "Having dense breast tissue alone doesn't make you more prone to developing cancer, though."

The primary risk factors for breast cancer include:

• Age — most are found in women over the age of 50
• A family history of breast cancer, particularly if an immediate family member was diagnosed at an early age
• Certain gene mutations, such as changes to the BRCA genes
  
  

Still, having more tissue for cancer to potentially grow in presents a risk. And it's one reason why mammogram results report on breast tissue density.

"Since there's more area for cancer to develop, adding a supplemental ultrasound on top of a screening mammogram is sometimes recommended," adds Dr. Mehta. "This additional screening can help ensure that a small finding isn't missed."

Is an ultrasound always needed if you have dense breast tissue?

The main reason it's important to know a person's breast density: This tissue can obscure small breast masses on a mammogram.

"The more dense breast tissue there is, the more challenging it becomes to read a mammogram," says Dr. Mehta. "It's not impossible, though, and advancements such as 3D technology have made mammograms even more accurate."

She adds that the recommendation for all women — even those with dense breasts — is still to get a screening mammogram every year starting at age 40.

"Mammograms are proven to be beneficial and are still considered the gold standard for breast cancer screening," explains Dr. Mehta. "But there's data to show that some women with dense breast tissue benefit from a supplemental ultrasound. There are cases where we find additional small breast cancers this way."

If your mammogram results indicate you have dense breast tissue, your physician will work with you to determine whether a supplemental ultrasound makes sense for your specific level of breast density, as well as how frequently one should accompany your annual screening mammogram.

(Related: Mammograms Vs. Ultrasound: Which Is Better?)

Can breast density change?

Yes.

In general, breast tissue becomes less dense over time with age. This means women in their 40s and 50s receiving supplemental ultrasounds because of dense breasts may need only a screening mammogram as their breasts become less dense later in life.

There's an exception, though.

"Often, postmenopausal women who take hormone replacement therapy will still have dense breast tissue as long as they're on the medication," adds Dr. Mehta.