Since the advent of the pink ribbon, women have been blasted with the message that mammograms save lives. And research does show that, in general, getting regular screening mammograms is generally the best way to catch breast cancer early.

But for some women, there's a catch: A certain type of breast tissue — known as dense breast tissue — makes it difficult to spot cancers on mammograms, because dense tissue appears white on this type of imaging test, as does cancer. What’s more, simply having dense breasts increases your cancer risk, says Beth A. Jones, PhD, MPH, a research scientist who studies cancer screening and prevention at Yale School of Public Health in New Haven, Connecticut.

Yet many women have never heard of dense breasts and don't know whether they have them.

What Are Dense Breasts?

Breast density has nothing to do with your bra size or how your breasts look or feel. It's not the same thing as having lumpy (fibrocystic) breasts either. If you have dense breasts, it means that you have a sizeable amount of fibrous or glandular tissue (versus fatty tissue) in your breasts. The only way discover this is by having a mammogram.

There are four categories that radiologists use to describe the density of breast tissue. You have dense breasts if you fall into the latter two categories:

Fatty Mostly fat tissue
Scattered Fibroglandular Mostly fatty with some fibrous and glandular tissue mixed in
Heterogeneously Dense Large areas of fibrous and glandular tissue
Extremely Dense Mostly fibrous and glandular tissue

Who Has Dense Breasts?

Having dense breasts isn't unusual, especially in young women. In fact, about one-half of women under age 50 have dense breasts. While density tends to decrease with age as estrogen levels decline, you can only expect a moderate shift, says Wendie Berg, MD, PhD, a radiologist at the University of Pittsburgh School of Medicine/UPMC. "If you were heterogeneously dense and go through menopause, you might move to scattered fibroglandular density. But you won't go from extremely dense to fatty." She estimates that about 25 percent of women in their sixties still have dense breasts.

Is There a Risk in Having Dense Breasts?

Having dense breasts isn't as big a risk factor as inheriting a genetic mutation like BRCA1 or BRCA2, but it's not insignificant: Studies show that if you have heterogeneously dense breasts, you're 1.62 times more likely than average to develop breast cancer. If your breasts are extremely dense, you're 2.04 times as likely to develop breast cancer. That's pretty similar to the increased risk conferred by having one first-degree relative with breast cancer (2.14 times the average risk), according to an article published in July 2017 in the journal Medical Clinics of North America. And research published September 2017 in the journal JAMA Oncology showed that dense breasts increase your risk of cancer more than other known risk factors, such as post-menopausal weight gain or a first pregnancy after age 35.

Dense breasts probably increase the risk of cancer, at least in part, because the glands that make up dense tissue tend to contain cells that frequently divide. Genetic mistakes, or mutations, are more likely to occur during cell division, so dividing more frequently gives these cells more of a chance to turn cancerous.

How to Find Out if You Have Dense Breasts

In the past, most women did not know if they had dense breasts, but that's starting to change. Laws vary, but at the moment 38 states have enacted legislation that requires women to be notified about their breast density after having a mammogram, per the website DenseBreast Info. Meanwhile, the U.S. Food and Drug Adminisitration (FDA) is currently developing a rule that will eventually require all patients in the United States to be notified if a mammogram shows that they have dense breasts, according to a proposal published in March 2019 in the Federal Register.

No matter where you live in the country, if you've had a mammogram the full report that goes to your doctor will nearly always say whether you have dense breasts. If you don't receive that information when you get your results, ask your doctor to obtain it.

Mammograms, Additional Screening, and Dense Breasts

If you do have dense breasts, you may want to opt for an ultrasound or MRI (magnetic resonance imaging) in addition to a mammogram. Don't skip mammograms, because they still detect many cancers, even in women with dense breasts, says Dr. Berg, whose breast cancer went undiagnosed initially because the mammogram missed the tumor because of dense breast tissue.

Berg, who has a strong family history of breast cancer, knew that she had dense breasts and pushed for an MRI that detected early stage breast cancer. She has since founded the website DenseBreast Info to educate both patient and clinicians about this issue.

Figuring out which supplemental test to have often comes down to your insurance coverage — MRIs tend to be more expensive than ultrasounds — and your overall risk of breast cancer. In making the decision for which test is appropriate, family history of cancer will also be taken into account.

Women who have a lifetime risk of breast cancer of at least 20 percent should get a mammogram and MRI every year, according to the American Cancer Society. But you should also talk to your doctor about the pros and cons of these tests. MRIs and ultrasounds are better at detecting cancer than mammograms, but they have a higher rate of false positives (test results that look positive for cancer but are proven wrong by subsequent tests).

Bottom line: If you have dense breasts and your doctor doesn't mention these additional tests, you should bring them up. "Shared decision-making is really critical," says Dr. Jones. "But patients need to be well-informed so they can advocate for themselves."

Dense Breasts 101: What You Need To Know if You Have This Type of Breast Tissue

What Are Dense Breasts?

Who Has Dense Breasts?

Is There a Risk in Having Dense Breasts?

How to Find Out if You Have Dense Breasts

Mammograms, Additional Screening, and Dense Breasts