Nearly half of all women who are 40 and older who get mammograms are found to have dense breast tissue. It’s long been known that women with dense breasts have a higher risk of getting breast cancer. But according to a new study reported in JAMA Oncology, the rate at which breast density changes over time also appears to be associated with a greater risk of breast cancer.

Breast structure is pretty complicated. They contain glandular tissue, fibrous connective tissue, and fatty breast tissue. Breast density is a term that describes the relative amount of these different types of breast tissue as seen on a mammogram, an X-ray picture of the breast that doctors use to look for early signs of breast cancer.

Breast density can be a Catch-22. Not only is it a known risk factor for breast cancer, dense breast tissue can make mammograms more difficult to read.

Dense breast tissue has relatively high amounts of glandular tissue and fibrous connective tissue and relatively low amounts of fatty breast tissue. As women age, it’s common for their breast density to lessen.

When a medical professional reads your mammogram, they will assess breast density along with other signs of cancer. They compare your previous mammograms to your most recent one, while looking for worrisome changes. But breast density can be a Catch-22. Not only is it a known risk factor for breast cancer, dense breast tissue can make mammograms more difficult to read.

To better understand the relationship of breast density to breast cancer, researchers from Washington University School of Medicine and Siteman Cancer Center at Barnes-Jewish Hospital in Saint Louis analyzed health data over the course of 10 years among 947 women recruited from November 2008 to April 2012 in the St. Louis region who completed routine mammograms through October 2020. The average age of the participants was 57.

There were 289 cases of diagnosed breast cancer among these women. The researchers found that breast density was higher at the start of the study for those women who developed breast cancer later in their lives, compared with those who remained cancer-free. That wasn’t surprising. But this was: The researchers also found that the rate of density decreasing over time was significantly slower among breasts in which cancer was later diagnosed.

New rules will require mammogram providers nationwide to notify women if they have dense breast tissue and recommend that they consult with a doctor about whether they need additional screening.

This is helpful diagnostic information. “By adding the change in density over repeated images to models for risk classification in each breast, we set the stage for better risk estimation with each updated mammogram. We can then better classify future risk and refer women to appropriate prevention strategies such as enhanced screenings as part of routine breast health services,” senior author, Graham A. Coldhitz, MD, DrPH, associate director of associate director for Prevention and Control at the Alvin J. Siteman Cancer Center at Washington University School of Medicine, said in a press statement.

If you’ve been told you have dense breasts, keep in mind that breast density is a modest risk factor. Women in the U.S. have a 1 in 8 lifetime chance of developing breast cancer. Women with dense breasts have only a slightly greater risk, about 1 in 6 or seven. So, the lifetime risk goes up from 12 percent to 15 percent. This means that most women with dense breasts will not develop breast cancer.

You can greatly reduce your risk of developing breast cancer according to the American Cancer Society by being physically active throughout your life, following a healthy eating pattern and avoiding or limiting alcohol.

When it comes to mammograms, the Food and Drug Administration (FDA) released new standards this past March that could potentially help more women detect breast cancer earlier. The new rules will require mammogram providers nationwide to notify women if they have dense breast tissue and recommend that they consult with a doctor about whether they need additional screening.