A functional ovarian cyst is a fluid-filled sac that forms on an ovary. A sac normally forms during ovulation to hold a maturing egg. Usually the sac goes away after the egg is released. But if an egg isn't released, or if the sac closes up after the egg is released, the sac can swell up with fluid.
During the normal monthly menstrual cycle, one of two types of functional cysts may form.
- A follicular cyst. This type occurs when a sac on the ovary doesn't release an egg and the sac swells up with fluid.
- A luteal cyst. This type occurs when the sac releases an egg and then reseals and fills with fluid.