(HealthDay News) -- Breast cysts are fluid-filled sacs that are formed when milk-producing glands become enlarged.

The California Pacific Medical Center says the cause of these cysts isn't fully understood, although it is believed that they are related to an irregular balance between fluid production and absorption.

Breast cysts are particularly common in women between the ages of 40 and 60. They are sometimes felt as lumps in the breast, but others are small enough to be detected only by a mammogram.

Most cysts in the breasts are benign, and usually do not mean an increased risk of breast cancer, the hospital says. But all cysts should be evaluated by a doctor without delay.