Ovarian cysts are very common. Each primordial follicle or egg cell is a potential cyst. Quite a few things can happen to cause a primordial follicle to develop into a cyst:

1. The follicle does not rupture and the egg cell is not released. In this case the follicle grows bigger and bigger and become a cyst. This kind of follicle can become very large.
   
2. The corpus luteum can grow abnormally large and bleeding can occur in the cavity inside corpus luteum and cause it to become a large cyst.
   
3. The cells surrounding the egg cell might began developing spontaneously and cause abnormal cyst formation. Some of these cyst can become cancer.
   
4. The egg cell may start to develop on its own and because it contains only half ( 23 instead of 46) of the human chromosomes abnormal growths and cyst formation can occur. (also known as a cystic teratoma or dermoid cyst)
   
5. Endometriosis can occur on the ovaries. It can infiltrate the ovarian tissue and can cause cysts. (For more information see Endometriosis).

Some cysts also produce abnormal quantities of female hormones (estrogen and progesterone) and can therefor cause menstrual abnormalities. (The lying of the womb (endometrium) is controlled by estrogen and progesterone . Refer to " menstruation" )

Not all cysts will cause menstrual abnormalities. They will only effect the menstrual cycle if they secrete hormones. Likewise ovulation is not necessarily effected by cysts.

Cysts are painful and potentially harmful conditions. They are an important cause of pain in the lower abdomen ( pain in area below the belly button). Medical advise should always be obtained in cases of abdominal pain.