Breasts described as "dense" have a high ratio of connective and glandular tissue to fat. On X-ray images, dense breast tissue looks solid and white, so it can mask tumors and make mammograms difficult to interpret. Increasingly, though, breast density is also being recognized as a breast cancer risk factor in itself. The mechanism behind this increased risk is unknown.
Your age and menopausal status affect your breast density. Younger women tend to have denser breasts. Hormones also have an effect — higher hormone levels generally mean denser breasts. Still, the actual increase in risk due to mammographic density is very small. If you're at high risk of breast cancer and your mammograms are difficult to interpret because of breast density, your doctor may recommend additional screening tests.