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A woman’s breast is made up of both fatty tissue and fibroglandular tissue (non-fatty) tissue. On a mammogram, fatty tissue appears black while the remaining breast tissue appears white or ‘dense’.

The amount of fibroglandular tissue (white areas) on a mammogram is referred to as breast density. Dense breasts are common and normal occurring in about 40% of women over 40 years or age.

As breast cancers also appear as white areas on a mammogram, high breast density reduces the ability to see breast cancers. Despite this, screening mammography is still the best population based breast cancer screening test for women aged 50 to 74, including those with dense breasts.

A series of four breast x-ray images, showing breast density ranges.

Breast density left to right: least dense to most dense.