Your ovaries normally grow cyst-like structures called follicles each month. Follicles produce the hormones estrogen and progesterone and release an egg when you ovulate.
Ovarian cysts are fairly common. Sometimes these follicles continue to grow and form a cyst. These are called functional cysts. There are two types of functional cysts, follicular cysts and corpus luteum cysts. A functional cyst occurs when LH (luteinizing hormone) is not released and the follicle does not release the egg and it turns into a cyst. These kinds of cysts are generally not harmful, do not cause pain, and go away in a few months. A corpus luteum cyst occurs when the LH is released and the ruptured follicle is now called the corpus luteum. There is an opening where the egg escapes but in this case it closes off and fluid builds up, causing a cyst. These kinds of cysts can be very painful and can grow to be up to four inches although they usually go away within a few weeks.
Ovarian cysts form for numerous reasons. The most common type is called a follicular cyst. A follicle is the fluid-filled sac that contains the egg. Follicular cysts form when the follicle grows larger than normal during the menstrual cycle and does not open to release the egg. Normally, follicular cysts resolve themselves over time.
Another menstrual cycle-related ovarian cyst is a corpus luteum cyst. The corpus luteum is an area of tissue within the ovary that appears after an egg has been released from a follicle. If a pregnancy doesn't occur, the corpus luteum usually breaks down and disappears. It may, however, fill with fluid or blood and develop as a cyst on the ovary. This cyst is typically found on only one side and has no symptoms.
Some other less common types of cysts that develop in the ovaries include cystic teratomas, dermoid cysts, and endometrioma.
There are at least two different types of cysts that can form in the ovaries, the first is called "functional cysts." These are actually formed normally during ovulation and also normally disappear after menstruation. Even if ovulation does not occur, they normally go away within a few months.
There are also non-functional, cancerous, or non-benign cysts though they make up just around five percent of the cysts that are formed in the ovaries. There are different causes for these different types of cysts including endometriosis.