There are different types of ovarian cysts. Functional cysts, which develop as a normal part of ovulation, are by far the most common type of ovarian cyst.

Each month a woman’s ovaries grow cyst-like structures called follicles or sacs. When an egg matures, the follicle breaks open to release the egg, which travels through the fallopian tube for fertilization. The sac is now called a corpus luteum cyst and persists to secrete hormones that support an early pregnancy. If a pregnancy does not occur, this cyst dissolves and stops producing these hormones. It is this drop in hormone levels that brings on menstruation.

Functional cysts are a product of normal ovarian function. There are two main types of functional cysts: 

• Follicular cyst. Develops before ovulation. These cysts are usually harmless, rarely cause pain and often disappear on their own within two to three menstrual cycles.
  
  
• Corpus luteum cyst. Develops after ovulation. Fluid can accumulate inside the sac, causing the cyst to grow, sometimes reaching up to 4 inches (10 centimeters) in diameter and can bleed into itself or twist the ovary, causing pain. If the cyst fills with blood, it can rupture, causing bleeding internally as well as a sudden, sharp pain.

A third type of follicle-related cyst, called theca luteum cyst, can develop during pregnancy but is quite rare. It may become quite large but also disappears on its own in most cases.

Polycystic ovaries is a term used to describe the enlarged ovaries with multiple, follicular cysts that sometimes form in women with polycystic ovarian syndrome (PCOS). To be considered polycystic, an ovary must have at least 12 follicular cysts in the range of ¼ to ½ inch (0.6 to 1.3 centimeters) in diameter. Not all women with PCOS develop polycystic ovaries.

There are other types of ovarian cysts, which are abnormal and much less common. They are usually benign but may be cancerous in some cases. They include:

• Dermoid cysts. Made of different kinds of tissue, such as skin, hair, fat and teeth. They are rarely cancerous, but can become large and twist the ovary causing pain. Ovarian cells can produce hair, teeth and other tissues that become part of the forming cyst.
  
  
• Cystadenoma cysts. Develop from cells on the outer surface of the ovary and are often filled with a watery fluid or thick, sticky gel. They can become large – 12 inches (30 centimeters) or more in diameter – and cause pain. They are divided into groups called serous or mucinous cystadenomas, depending on certain characteristics.
  
  
• Endometrioma cysts (also called chocolate cysts because they are filled with dark, reddish-brown blood). Endometrioma cysts develop in women who have endometriosis (when tissue from the lining of the uterus grows outside of the uterus). The tissue may attach to the ovary and form a growth. They can be painful during menstruation and sexual intercourse.