A teratoma is a tumor with tissue or
 components resembling normal derivatives of more than one germ layer. Although the teratoma may be monodermal or polydermal (originating from one or more germ layers), its cells may differentiate in ways suggesting other 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 or processes such as brain matter ,
eyes ,torso, and hands, feet , or other limbs .
Usually, 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 often remain undiscovered until much later in life.

Signs and Symptoms

 Teratoma of germ cell origin usually are found in adult men and women, but they may also be found in children and infants. Teratomas of embryonal origin are most often found in babies at birth, in young children, and, since the advent of ultrasound imaging , in fetuses.
The most commonly diagnosed fetal teratomas are sacrococcygeal teratoma (Altman types I, II, and III) and cervical (neck) teratoma. Because these teratomas project from the fetal body into the surrounding amniotic fluid , they can be seen during routine prenatal ultrasound exams. Teratomas within the fetal body are less easily seen with ultrasound.
Teratomas are not dangerous for the fetus unless there is either a mass effect or a large amount of blood flow through the tumor (known as vascular steal). The mass effect frequently consists of obstruction of normal passage of fluids from surrounding organs. The vascular steal can place a strain on the growing heart of the fetus, even resulting in heart failure, and thus must be monitored by fetal
After surgery, there is a risk of regrowth in place, or in nearby organs.

Some terms that have been synonymous with teratoma include dysembryoma, teratoblastoma ,
organoid tumor , and teratoid tumor .
The words "teratoma" and "mature teratoma" both have been used to refer to a benign growth, while the word "teratoma" may also refer to "immature teratoma", a cancerous growth.