Uterine fibroids often appear in clusters, although in rare instances only a single fibroid may develop. They generally form in the wall of the uterus (womb), but also may grow into the cervix. These benign tumors range in size from tiny, undetectable specks to large, bulky masses. Large fibroid masses left untreated may compromise a woman’s reproductive health.

Fibroids are classified depending on their location in the uterus:

• Intramural fibroids. These fibroids develop in the muscular layers of the uterine wall (myometrium). Intramural fibroids are the most common type of fibroids.
  
  
• Subserosal fibroids. The second most common type, subserosal fibroids grow in the outer wall of the uterus.
  
  
• Submucosal fibroids. The least common form of fibroids, they protrude into the uterine cavity.
  
  
• Pedunculated fibroids. Sometimes a subserosal or submucosal fibroid develops a stalk (peduncle), similar to the stem of a plant, and hangs from the uterus. These fibroids are known as pedunculated, and as they get bigger they may twist on their stalks, causing severe pain.
  
  
• Parasitic fibroids. A rare condition that occurs when a uterine fibroid attaches itself to another organ.

It is not uncommon for women to have more than one type of uterine fibroids.