The monthly variations in the circulating levels of estrogen and progesterone are thought to account for most fibrocystic breast changes. Although the exact contribution of each hormone is not well understood, it is believed that an excess amount of estrogen over progesterone results in edema of the breast tissue. At the onset of menses, hormone levels decrease and the fluid responsible for the breast edema is removed by the lymphatic system. All the fluid in the breast may not be removed, and eventually, the fluid accumulates in the small glands and ducts of the breast, allowing cyst formation.
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