The next time you have a mammogram, take the opportunity to ask your doctor if you have dense breasts. The answer to this question may help prevent you from being among the nearly 40,000 American women who die of breast cancer each year.

“Knowledge is power when it comes to your health,” says Dr. Cristina Vieira, a radiologist at Baptist Health Breast Center.

When your doctor receives your mammogram report from the radiologist, it provides information about the density of your breasts. The more fibro-glandular tissue you have in your breasts compared to fatty tissue, the denser your breasts.

Remember that breasts are glands that produce milk. They contain lobules that are little sacks where the milk is formed, ducts which transport the milk, and tissue surrounding the glandular structures that give the breast its form and shape. These make up the fibro-glandular portion of your breasts, explains Dr. Monica Yepes, a breast radiologist at the University of Miami Sylvester Comprehensive Cancer Center.

Whenever you have a mammogram, the radiologist reading the X-rays determines the density of your breasts by placing them on a scale of one (least dense) to four (most dense) using the Breast Imaging Reporting and Data System (BI-RADS) of the American College of Radiology.

About half of all women are in categories three and four and therefore have dense breasts. “If you fall into one of these categories, don’t worry, it’s normal,” Yepes says. “It’s just how your body is made up.”

And your breast density can change over your lifetime. Younger women and older women on hormone replacement therapy tend to have more fibro-glandular tissue and denser breasts than others. Also a large weight loss or gain may also affect breast density.

But one thing doctors know for sure is that dense breasts do not increase your risk of dying from breast cancer.

Dr. Mary Hayes, chief of Women’s Imagine for Radiology Associates of Florida, explains that if two patients are identical except one has dense breasts, both women will respond the same way to the same cancer.

“However the trick is to find the cancer in that dense breast tissue,” says Hayes who works at Memorial Healthcare Systems.

When she looks at the film from a mammogram, the fatty breast tissue appears gray or black on the X-ray. Meanwhile, the fibro-glandular tissues as well as many cancers appear white.

“So detecting a cancer among the dense tissue is like looking for a snowball in a snowstorm or the stars in the daytime sky,” Hayes says. “You know the stars are there but it’s much easier to see them against the dark night sky.”

Although mammography is still the gold standard for detecting breast cancers and has reduced the likelihood of dying from breast cancer by 30 percent according to Hayes, it’s not a perfect tool. So for patients with dense breasts in particular, your doctor may recommend further screening.

One option that gained FDA approval in February 2011, is 3-D mammography or tomosynthesis.

The machine looks much like the one used for traditional mammography but instead of taking one picture, it produces an image that resembles the pages of a book. Each “page” represents a one millimeter thickness of your compressed breast.

“If there is any dense breast tissue or other features that are causing the potential cancer to hide, we can unravel them,” Hayes says.