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.
Sometimes a normal monthly follicle just keeps growing. When that happens, it becomes known as a functional cyst. This means it started during the normal function of your menstrual cycle. There are two types of functional cysts:
• Follicular cyst. Around the midpoint of your menstrual cycle, your brain's pituitary gland releases a surge of luteinizing hormone (LH), which signals the follicle holding your egg to release it. When everything goes according to plan, your egg bursts out of its follicle and begins its journey down the fallopian tube in search of sperm and fertilization.
A follicular cyst begins when the LH surge doesn't occur. The result is a follicle that doesn't rupture or release its egg. Instead it grows and turns into a cyst. Follicular cysts are usually harmless, rarely cause pain and often disappear on their own within two or three menstrual cycles.
• Corpus luteum cyst. When LH does surge and your egg is released, the ruptured follicle begins producing large quantities of estrogen and progesterone in preparation for conception. This changed follicle is now called the corpus luteum. Sometimes, however, the escape opening of the egg seals off and fluid accumulates inside the follicle, causing the corpus luteum to expand into a cyst.
Although this cyst usually disappears on its own in a few weeks, it can grow to almost 4 inches in diameter and has the potential to bleed into itself or cause the ovary to twist, cutting off its blood supply and causing pelvic or abdominal pain. If it fills with blood, the cyst may rupture, causing internal bleeding and sudden, sharp pain. The fertility drug clomiphene citrate (Clomid, Serophene), which is used to induce ovulation, increases the risk of a corpus luteum cyst developing after ovulation. These cysts don't prevent or threaten a resulting pregnancy.