A functional ovarian cyst is caused by one or more slight changes in the way the ovary produces or releases an egg. During the normal monthly menstrual cycle, one of two types of functional cysts may develop:

• A follicular, or simple, cyst occurs when the small egg sac (follicle) on the ovary does not release an egg, and it swells with fluid either inside the ovary or on its surface.
• A luteal, or corpus luteum, cyst occurs when the remains of the egg follicle do not dissolve and continue to swell with fluid. This is the most common type of ovarian cyst.

The development of luteal cysts is also common during treatment with clomiphene citrate (such as Clomid or Serophene) for infertility. These cysts go away after treatment is completed, though this can take several months. They do not appear to endanger pregnancy. For more information, see the topic Fertility Problems.

Other ovarian growths

The development of functional cysts is directly related to ovulation. However, there are other types of ovarian cysts and growths caused by other conditions. An ovarian growth can be a noncancerous (benign) cystic tumor or related to endometriosis or cancer. In some cases, what seems to be an ovarian mass is actually growing on nearby pelvic tissue. This is why it's important for you to have regular pelvic exams and for your health professional to carefully diagnose any cysts or growths felt on your ovaries.