Compression holds the breast still to reduce blurring, it spreads out overlapping tissue for clearer images and reduces thickness of tissue. Little is known about the relationship between the amount of breast compression and breast cancer detectability. 

Most mammography systems apply a compression force between 100–200 Newtons (N), roughly equivalent to pressing with 10–20 kg or 22-45 lbs of weight. The less compression (and therefore less pain), the more radiation is required for images.   

Compression thickness (how “flat” the breast becomes) is also tracked. A typical compressed breast is 4–6 cm thick, but can be thinner or thicker depending on breast size and density.  Some machines are automated and stop at a preset pressure level, while others rely on the technologist’s judgment.  Different technicians may use different amounts of force depending on training, patient tolerance, and habit. How does this translate to pain and image clarity? More compression = better image clarity, lower radiation dose, but more pain. Less compression = more comfortable, but higher chance of blurry or overlapping tissue, false positives, and higher radiation. 

Compression can lead to temporary bruising, swelling, or tenderness, and in some cases has been linked to hematomas (internal bleeding in breast tissue) and complications for women with implants, including reports of rupture submitted to the FDA. Women with small breasts may experience more pain because the same force is applied over a smaller surface area, while women with very large or dense breasts can suffer uneven compression and repeated repositioning. If there is already inflammation, a lump, or a recent biopsy, compression can worsen sensitivity. Though usually described as a “necessary discomfort,” the reality is that mammographic compression can cause real physical harm in addition to temporary pain. This pain isn’t just uncomfortable—it can signal real harm.  

There is sufficient evidence to conclude that painful mammography contributes to non-re-attendance.