An ovarian cyst is a fluid-filled sac in the ovary. During the menstrual cycle, it is normal for a cyst to develop. Most cysts are small and benign (not cancerous) and go away on their own. Larger cysts can cause pain and other problems.

Close Up of Ovary and Fallopian Tube
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Follicles grow in the ovaries each month during childbearing years. Each month, in an ovary, at least one egg matures in its follicle. The egg and follicle become a small functional cyst. It makes hormones and during ovulation will release an egg. In some cases, these follicles may enlarge enough to become cysts. They are also known as functional cysts. There are two main types:

  • Follicular cyst—This occurs when a follicle does not mature properly. The egg is not released. The follicle and egg develop into a cyst. This type of cyst goes away on its own within 1-3 menstrual cycles.
  • Corpus luteum cyst—This occurs after an egg is released from the follicle during ovulation. Fluid builds up in the follicle and creates a cyst. This type of cyst goes away on its own in a few weeks.

Other, less common types of benign cysts can also form from ovarian tissue:

  • Dermoid cyst—This cyst is made up of tissue from other parts of the body.
  • Endometrioma—Endometrial tissue (lining of uterus) appears to be able to move away from the uterus to the ovary. Cysts may grow and become filled with fluid (often blood).
  • Cystadenoma—This cyst grows from cells that line the outside of the ovary. Cystadenoma can become large and painful.

In a small number of cases, some cysts undergo cancerous changes. The doctor will carefully examine each cyst.