A teratoma is an encapsulated tumor with tissue or organ components resembling normal derivatives of all three germ layers. The tissues of a teratoma, although normal in themselves, may be quite different from surrounding tissues and may be highly disparate; teratomas have been reported to contain hair, teeth, bone and, very rarely, more complex organs such as eyes, torso, and hands, feet, or other limbs.
Usually, however, a teratoma will contain no organs but rather one or more tissues normally found in organs such as the brain, thyroid, liver, and lung. Sometimes, the teratoma has within its capsule one or more fluid-filled cysts; when a large cyst occurs, there is a potential for the teratoma to produce a structure within the cyst that resembles a fetus. Because they are encapsulated, teratomas are usually benign, although several forms of malignant teratoma are known and some of these are common forms of teratoma. A mature teratoma is typically benign and found more commonly in women, while an immature teratoma is typically malignant and is more often found in men.
Teratomas are thought to be present at birth (congenital), but small ones are often not discovered until much later in life.
My niece had one that was removed from her ovary when she was in here late 20's or early 30's.
undeveloped twin?