Harvard Women's Health Watch

Why breast density matters

What it means to have dense breasts—and how they can affect your breast cancer risk.

After a routine mammogram, your radiologist or doctor might have told you that you have dense breasts. But what exactly are dense breasts, and how can they affect your breast cancer risk?

Simply put, "Women with dense breasts have more mammary tissue than fatty tissue," explains Dr. Robyn Birdwell, associate professor of radiology at Harvard Medical School, and section head in the division of breast imaging at Brigham and Women's Hospital. If more than 50% of your breasts is made up of connective and glandular breast tissue (as opposed to fat), then you have dense breasts. About 40% of women have dense breasts.

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