Breast Density Categories: A Through D, Explained

What are the four density categories?

Radiologists grade breast tissue into four density levels labeled A through D, describing how much fibroglandular tissue appears compared with fat.

• A - Almost entirely fatty: very little dense tissue, easiest to read
• B - Scattered density: mostly fat with some fibroglandular areas
• C - Heterogeneously dense: enough dense tissue to hide small masses
• D - Extremely dense: mostly dense tissue, hardest to read

Under the American College of Radiology system, the grade appears on your report next to the BI-RADS 0-to-6 category. According to the National Cancer Institute, this scale describes the appearance of the tissue, not disease 1Ref 1National Cancer Institute (PDQ Screening and Prevention Editorial Board) (2025).Breast Cancer Screening.NCI patient screening page supporting that the density scale describes tissue appearance, that dense breasts mask cancers and modestly raise risk, and that higher-risk women may be offered supplemental imaging. The letters run in order of increasing density, so A and D mark the two ends and B and C fill the common middle.

Which grades count as 'dense'?

Categories C and D are the two grades considered dense on a mammogram report, while A and B are not. Heterogeneously dense tissue, category C, has enough white fibroglandular tissue to obscure small masses, and extremely dense tissue, category D, lowers a mammogram's sensitivity the most. According to the National Cancer Institute, dense breasts both mask cancers and modestly raise risk 1Ref 1National Cancer Institute (PDQ Screening and Prevention Editorial Board) (2025).Breast Cancer Screening.NCI patient screening page supporting that the density scale describes tissue appearance, that dense breasts mask cancers and modestly raise risk, and that higher-risk women may be offered supplemental imaging. That is why a C or D grade triggers the density notification on your mammogram letter.

According to the American College of Obstetricians and Gynecologists, density is one factor among several in screening decisions 2Ref 2American College of Obstetricians and Gynecologists (2017).Practice Bulletin Number 179: Breast Cancer Risk Assessment and Screening in Average-Risk Women.ACOG guidance that density is one factor among several in screening decisions, is a radiologist's visual assessment, and shifts with hormones, menopause, and weight. The A-to-B range is the reassuring, easier-to-read end of the scale. Roughly speaking, the more white fibroglandular tissue shows on the image, the higher the density letter climbs.

How does the radiologist decide?

A radiologist assigns the density grade by eye, comparing the amount of white fibroglandular tissue with the darker fat on your images. Because it is a visual judgment, two readers can occasionally land on neighboring grades such as B versus C, and the same person's grade can shift year to year. According to the American College of Obstetricians and Gynecologists, density assessment is one input clinicians weigh rather than a fixed lab value 2Ref 2American College of Obstetricians and Gynecologists (2017).Practice Bulletin Number 179: Breast Cancer Risk Assessment and Screening in Average-Risk Women.ACOG guidance that density is one factor among several in screening decisions, is a radiologist's visual assessment, and shifts with hormones, menopause, and weight. Automated software is increasingly used to make the call more consistent. Mammography and its density grade also fit into your broader cancer screenings by age. Because the grade is comparative rather than a precise measurement, a one-step change between years rarely signals anything real.

How does density change over time?

Breast density is not fixed and often shifts across the lifespan. Density tends to be higher in younger, premenopausal women, when tissue is more glandular, and it typically decreases with age as more of the breast becomes fatty. According to breast-screening guidelines, hormone therapy can raise density, while menopause and weight gain tend to lower it 2Ref 2American College of Obstetricians and Gynecologists (2017).Practice Bulletin Number 179: Breast Cancer Risk Assessment and Screening in Average-Risk Women.ACOG guidance that density is one factor among several in screening decisions, is a radiologist's visual assessment, and shifts with hormones, menopause, and weight. That is why your grade may move between categories over the years even though nothing is wrong.

Comparing your density with your mammogram screening schedule helps put a single year's grade in context. A shift of one category is common and usually not concerning. About 4 in 10 women fall into the dense C or D range, and according to the American College of Obstetricians and Gynecologists that share drops with age 2Ref 2American College of Obstetricians and Gynecologists (2017).Practice Bulletin Number 179: Breast Cancer Risk Assessment and Screening in Average-Risk Women.ACOG guidance that density is one factor among several in screening decisions, is a radiologist's visual assessment, and shifts with hormones, menopause, and weight.

When your density grade needs a conversation

A category C or D grade, combined with other risk factors such as family history, is a reason to discuss screening options with a clinician. According to the National Cancer Institute, higher-risk women may be offered supplemental imaging alongside mammography 1Ref 1National Cancer Institute (PDQ Screening and Prevention Editorial Board) (2025).Breast Cancer Screening.NCI patient screening page supporting that the density scale describes tissue appearance, that dense breasts mask cancers and modestly raise risk, and that higher-risk women may be offered supplemental imaging. Noticing a lump or change while checking your breasts is worth reporting regardless of your grade. Gale can help you gather your history and prepare the questions that make a density conversation more productive. Knowing your grade beforehand lets you ask whether extra imaging would change anything. According to the National Cancer Institute, about 1 in 8 women in the United States develop breast cancer over a lifetime 1Ref 1National Cancer Institute (PDQ Screening and Prevention Editorial Board) (2025).Breast Cancer Screening.NCI patient screening page supporting that the density scale describes tissue appearance, that dense breasts mask cancers and modestly raise risk, and that higher-risk women may be offered supplemental imaging, and screening mammograms are generally offered from age 40, every 1 to 2 years, to at least age 75 2Ref 2American College of Obstetricians and Gynecologists (2017).Practice Bulletin Number 179: Breast Cancer Risk Assessment and Screening in Average-Risk Women.ACOG guidance that density is one factor among several in screening decisions, is a radiologist's visual assessment, and shifts with hormones, menopause, and weight.