Fibroglandular density refers to the proportion of dense tissue versus fatty tissue in your breast, as seen on a mammogram. Every breast contains a mix of two main tissue types: fibroglandular tissue (a combination of supportive connective tissue and the glandular cells that line the milk ducts) and fat. When a radiologist describes your fibroglandular density, they’re telling you how much of your breast is made up of that dense, non-fatty tissue. Nearly half of women aged 40 and older who get mammograms have dense breast tissue.
What Fibroglandular Tissue Actually Is
Your breast is built from two basic components. The first is fibroglandular tissue, which itself is a combination of two things: fibrous connective tissue (called stroma) that provides structural support, and glandular epithelial cells that line the ducts. These glandular cells are the functional part of the breast, responsible for producing and transporting milk. The second component is fat, which fills the space around and between the fibroglandular tissue.
On a mammogram, fibroglandular tissue appears white or light gray, while fat appears dark. The ratio between these two tissue types is what determines your breast density classification. This ratio varies widely from person to person and can change over your lifetime.
The Four Density Categories
Radiologists classify breast density into four standardized categories, labeled A through D:
- Category A: Almost entirely fatty, with less than 25% glandular tissue.
- Category B: Scattered areas of fibroglandular density, roughly 25% to 50% glandular tissue.
- Category C: Heterogeneously dense, meaning about 51% to 75% glandular tissue. Small masses may be harder to spot.
- Category D: Extremely dense, with more than 75% glandular tissue. This significantly lowers mammography’s ability to detect abnormalities.
Categories C and D are considered “dense.” About 40% of women fall into category C, and roughly 10% fall into category D. If your mammogram report mentions “scattered areas of fibroglandular density,” you’re in category B, which is not classified as dense.
Why Density Matters on a Mammogram
Both fibroglandular tissue and tumors appear white on a mammogram. This creates what radiologists call a masking effect: the denser your breast tissue, the easier it is for a small cancer to hide behind or within that white tissue, like trying to spot a snowball in a snowstorm. The thicker the layer of dense tissue at any given spot, the higher the chance a lesion in that area goes undetected. Larger tumors are harder to mask, but small, early-stage cancers are the ones most likely to be missed.
This is why mammography becomes less sensitive as density increases. In extremely dense breasts (category D), the sheer volume of white tissue on the image can obscure cancers that would be plainly visible in a mostly fatty breast.
Density and Breast Cancer Risk
High fibroglandular density is one of the strongest known risk factors for breast cancer, independent of its effect on screening accuracy. Women in the highest density category face four to six times the risk of breast cancer compared to women in the lowest category. Even a moderately elevated density carries meaningful risk: women with density above 51% have roughly double the risk of those with average density (11% to 25%).
It’s worth noting that density is a risk factor, not a diagnosis. Most women with dense breasts will never develop breast cancer. But the combination of increased biological risk and reduced mammographic sensitivity is what makes density such an important piece of information.
What Influences Your Density
Several factors shape how dense your breasts are, and some of them shift over time.
Hormones play the biggest role. Progesterone drives breast cell proliferation and is strongly linked to higher density. Women taking combined hormone therapy, particularly regimens containing a progestin, tend to see their density increase. This is one reason density often decreases after menopause, when hormone levels drop, though paradoxically, a longer time since menopause has been associated with increased density in some studies.
Body weight also has a clear effect, but in the opposite direction from what many people expect. A higher BMI is associated with lower breast density, because larger breasts in women with higher BMI tend to contain proportionally more fat. Having more pregnancies and more full-term births also reduces density. On the flip side, having fewer pregnancies and a lower BMI are independent risk factors for higher density. Genetics and ethnicity play a role too, with breast density showing variation across geographic and ethnic populations. Smoking and physical activity, interestingly, do not appear to significantly change density.
What Happens After a Dense Breast Finding
As of 2024, the FDA requires all mammography facilities in the United States to notify patients about their breast density in their results letter. The notification explains how density can affect the accuracy of mammography and recommends that women with dense breasts discuss their individual risk factors and screening options with a provider.
For women in categories C and D, supplemental screening may add a meaningful layer of detection. Breast ultrasound can find cancers that mammography misses in dense tissue, and breast MRI is even more sensitive, though it comes with a higher rate of false positives and is typically reserved for women who also have other elevated risk factors, such as a strong family history or known genetic mutations. Contrast-enhanced mammography is a newer option gaining availability. The right approach depends on your overall risk profile, not just your density category alone.
Density Can Change Over Time
Your fibroglandular density is not fixed. It tends to be highest during your reproductive years and gradually decreases after menopause as glandular tissue is replaced by fat. Weight gain, pregnancy, and stopping hormone therapy can all shift your density downward. Conversely, starting hormone therapy or losing significant weight can increase it. Because of these fluctuations, your density category can change from one mammogram to the next, and your screening approach may need to adapt accordingly.