Fetus in fetu differs from fetiform teratoma in having an apparent spine and bilateral symmetry.Most authorities agree that fetiform teratomas are highly developed mature teratomas;  the natural history of fetus in fetu is controversial. There also may be a cultural difference, with fetiform teratoma being reported more often in ovarian teratomas (by gynecologists) and fetus in fetu being reported more often in retroperitoneal teratomas (by general surgeons). Fetus in fetu has often been interpreted as a fetus growing within its twin. As such, this interpretation assumes a special complication of twinning, one of several grouped under the term parasitic twin. In this regard, in many cases the fetus in fetu is reported to occupy a fluid-filled cyst within a mature teratoma. Cysts within mature teratoma may have partially developed organ systems; reports include cases of partial cranial bones, long bones and a rudimentary beating heart.Regardless of whether fetus in fetu and fetiform teratoma are one entity or two, they are distinct from and not to be confused with ectopic pregnancy. Struma ovarii
A struma ovarii (also known as goitre of the ovary or ovarian goiter) is a rare form of mature teratoma that contains mostly thyroid tissue. Epignathus
Epignathus is a rare teratoma originating in the oropharyngeal area that occurs in utero. It presents with a mass protruding from the mouth at birth. Untreated, breathing is impossible. An EXIT procedure is the recommended initial treatment. Signs and symptoms
Teratomas may be found in babies, children, and adults.
The kind of pluripotent cell appears to be unimportant, apart from constraining the location of the teratoma in the body. Teratomas derived from germ cells occur in the testicle and ovaries. Teratomas derived from embryonic cells usually occur on the subjects midline: in the brain, elsewhere in the skull, in the nose, in the tongue, under the tongue, and in the neck (cervical teratoma), mediastinum, retroperitoneum, and attached to the coccyx. Teratomas may also occur elsewhere: very rarely in solid organs (most notably the heart and liver) and hollow organs (such as the stomach and bladder), and more commonly on the skull sutures. Teratoma rarely include more complicated body parts such as teeth, brain matter, eyes, or torso. Hypotheses of origin
Concerning the origin of teratomas,  numerous hypotheses exist. These hypotheses are not to be confused with the unrelated hypothesis that fetus in fetu (see below) is not a teratoma at all, but rather a parasitic twin. Diagnosis
Teratomas are thought to originate in utero, so can be considered congenital tumors. Many teratomas are not diagnosed until much later in childhood or in adulthood. Large tumors are more likely to be diagnosed early on. Sacrococcygeal and cervical teratomas are often detected by prenatal ultrasound. Additional diagnostic methods may include prenatal magnetic resonance imaging. In rare circumstances, the tumor is so large that the fetus may be damaged or die.