A parasitic twin is the result of a situation related to the process that results in teratomas, vanishing twin syndrome and conjoined twins—two unique embryos begin developing in utero, but something goes wrong. Parasitic twins are also known as asymmetrical conjoined twins or unequal conjoined twins. Parasitic twins are a variation on conjoined twins—except one of the twins stopped developing during the gestation process and is now vestigal to a healthy, otherwise mostly fully formed individual twin. They are defined as parasitic, rather than conjoined, by being incompletely formed or wholly dependent on the body functions of the complete fetus.
Conjoined-parasitic twins united at the head are described as craniopagus or cephalopagus. Craniopagus occipitalis is the term for fusion in the occipital region, craniopagus parietalis is is when the fusion is in the parietal region, craniopagus parasiticus is term for a parasitic head attached to the head of a more fully developed fetus or infant.
Some parasitic twins live inside their host twin creating a fetus in fetu. These can survive even after the host is born until they receive medical intervention.
See also: Fetus in fetu, Acardiac twin