Breast density B, formally called “scattered areas of fibroglandular density,” means your breasts are mostly fatty tissue with some patches of denser tissue mixed in. It’s the second of four density categories assigned after a mammogram, and it’s the most common classification. About 38% of women fall into this category. If you just received this on your mammogram report, it’s considered “not dense” and is generally reassuring.
What Category B Looks Like on a Mammogram
Breast tissue is made up of two main components: fatty tissue and fibroglandular tissue (a mix of connective tissue and milk-producing glands). On a mammogram, fatty tissue appears dark or translucent, while fibroglandular tissue shows up as solid white. A radiologist reading your images can easily see through the dark areas but not through the white ones.
With Category B density, most of the breast appears dark on the image, but there are scattered white patches throughout. This is different from Category A (almost entirely fatty, with very little white), Category C (heterogeneously dense, where roughly half or more of the breast is white), and Category D (extremely dense, where nearly the entire breast is white). Categories C and D are classified as “dense breasts,” while A and B are classified as “not dense.”
How the Four Density Categories Compare
- Category A: Almost entirely fatty tissue. About 33% of women. Mammograms are highly effective here.
- Category B: Mostly fatty with scattered dense areas. About 38% of women. Mammograms still work well.
- Category C: Heterogeneously dense. About 24% of women. Mammogram sensitivity starts to drop.
- Category D: Extremely dense. About 5% of women. Mammogram sensitivity can fall as low as 25% to 30%.
What This Means for Cancer Risk
Higher breast density is associated with a higher likelihood of developing breast cancer, though the reasons aren’t fully understood. The relationship follows a gradient: the denser your breasts, the greater the statistical risk. Category B sits near the lower end of that spectrum. Your risk is slightly higher than someone with entirely fatty breasts (Category A), but meaningfully lower than someone with heterogeneously or extremely dense tissue (Categories C or D).
Dense tissue itself may play a biological role in cancer development, not just a masking role. But for women in Category B, the overall picture is relatively favorable. The scattered dense patches aren’t extensive enough to substantially elevate risk on their own.
How Well Mammograms Work at This Density
One of the biggest practical concerns with breast density is whether tumors can hide behind the white areas on a mammogram. For women with fatty or scattered fibroglandular tissue (Categories A and B), mammograms approach nearly 100% sensitivity, meaning they catch almost all cancers present. That number drops significantly for dense breasts, falling to as low as 25% to 30% in extremely dense tissue.
Because Category B is classified as “not dense,” standard mammography is considered effective for you. Major medical organizations, including the American College of Obstetricians and Gynecologists, do not recommend supplemental screening tests like ultrasound or MRI for women with scattered fibroglandular density and no other risk factors. If your mammogram report says Category B, routine screening at whatever interval your provider has recommended is the standard approach.
What Influences Your Breast Density
Breast density isn’t fixed. It shifts over time based on several factors, and it’s possible to move between categories from one mammogram to the next.
Age and menopause are the biggest drivers. Breasts tend to become less dense as you get older, with fatty tissue gradually replacing fibroglandular tissue. Many women who were Category C in their 40s shift to Category B after menopause. Body weight also plays a role: higher BMI is associated with lower breast density, because the breasts contain more fatty tissue. The number of pregnancies you’ve had has a similar effect, with more pregnancies linked to reduced density over time.
Hormones push density in the other direction. Longer exposure to progesterone is associated with increased density, and postmenopausal hormone therapy can raise density as well. This is one reason why the same woman might see her density category change depending on whether she’s using hormone therapy. If you start or stop hormone therapy, your density classification could shift on your next mammogram.
What to Do With This Information
If your mammogram report says Category B, no additional screening beyond standard mammography is typically needed based on density alone. Your results fall into the “not dense” range, mammograms work well for your tissue type, and your density-related cancer risk is on the lower end. That said, breast density is just one piece of your overall risk profile. Family history, genetic factors, and other personal health details all matter independently of what your tissue looks like on an image. Your density category is useful context, not the whole picture.