Photo: Getty Images/Larry Washburn When you think about breast cancer screening, one word usually comes to mind: mammogram. But for about half of the women in this country, this traditional measure of testing might not be enough.

That's because roughly 50 percent of women in the U.S. have what's called dense breasts, a description that refers to the amount of fibroglandular tissue versus fat in the breast—a factor that impacts both cancer risk and detection, explains Laurie Margolies, M.D., the chief of breast imaging at the Mount Sinai Health System in New York. (Related: 5 Simple Ways to Reduce Your Breast Cancer Risk)

Women with the densest breasts are five to six times more likely to develop breast cancer than women with the lowest breast density, Dr. Margolies says. Why? It could be that with dense breasts, there's more active tissue that can develop breast cancer, she explains. "The fatty part typically doesn't develop breast cancer. With dense breasts, there are more opportunities for something to go wrong as a cell divides."

The other biggie: Mammograms can miss cancers in women with dense breasts. Fatty breast tissue is ideal for a mammogram. "The cancer shows as a white spot and it's easy to see in a gray background," explains Dr. Margolies. An extremely dense breast, on the other hand, appears on a mammogram like a white piece of paper. "The cancer is white and the tissue is white, so the cancer is much harder to see unless there is some fat around tissue," she says. "Dense breast tissue is tissue that might hide a small cancer—or a not-so-small cancer."

Surprised? You're not alone. A survey published in Oncology Times this year discovered that only 20 percent of women realize breast density plays a role in how well a mammogram detects cancer; even fewer ladies were aware that dense breast tissue was a risk factor for breast cancer.

So now, the important stuff: How do you know if your breasts are considered "dense"?

For one, younger women are generally more likely to have dense breasts than older women. "Breast density is related to the hormonal milieu that the body has," says Dr. Margolies. "As we hit menopause, fatty tissue increases and fibroglandular tissue decreases." From an evolutionary standpoint, that's likely because fibroglandular tissue is needed to produce milk, and as we hit menopause we no longer need it. Breast density will likely be found to have a genetic component, too, notes Dr. Margolies: "But no one yet knows what it is or how to test for a potential breast density gene." (Related: This Company Offers Genetic Testing for Breast Cancer at Home)

But you can't figure out your breast density by your age or even through a self-check. When it's time to get your first mammogram (age 40 or 45 depending on your risk factors—talk to your doctor about the best age for you), the radiologist will typically record your breast density in their radiology report.

Historically, women were not told this was in their report, says Margolies. But now 30 states require that if your breasts are dense, that information be included in your results. Other states, like Florida, have bills pending on the topic, meaning that the number of states where this is a requirement could climb.

When the first breast density law went into effect in Connecticut, a number of health professionals worried these laws would lead to unnecessary procedures, false positives, and patient anxiety. And, Dr. Margolies confirms, initial results did indeed show that there were many false positives and many benign biopsies completed. But, she adds: "In the ensuing eight years, this has dramatically improved."

Today, breast density awareness is seen as a lifesaver. In part, that's because if you have dense breasts, getting an ultrasound—a supplementary breast cancer screening for women with dense breasts—can catch a cancer that a mammogram might miss. Says Dr. Margolies: "If you take 1,000 women with dense breasts who had a normal mammogram, three will have a tiny, little invasive breast cancer that an ultrasound can pick up early."

That might not seem like a big number, but considering breast cancer takes the lives of 40,610 women every year, catching the cases that might otherwise be missed proves incredibly important. For now, make sure to check out the laws in your state regarding breast density—and always ask questions if you're unsure. It could be the difference between catching a cancer early and missing it until it's too late.