Parasitic twin

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A parasitic twin is the result of a situation related to the process that results in vanishing twin 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 gestation and is now vestigial to a healthy, otherwise mostly fully-formed individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is 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 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.

1 Specific types of parasitic twin
- 1.1 Acardiac twin or TRAP sequence
2 See also
- 2.1 Fiction
3 References
4 External links

[edit] Specific types of parasitic twin

[edit] Acardiac twin or TRAP sequence

An acardiac twin, also called the TRAP sequence, is a parasitic twin that fails to develop a head, arms and a heart. The resulting torso survives by leeching blood flow from the surviving normal twin by means of an umbilical cord-like structure, much like a fetus in fetu, except the acardiac twin is not enveloped inside the normal twin's body. Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus's heart. This twinning condition usually occurs very early in pregnancy.

Without intervention, the normal fetus is only expected to have a 20% chance of survival. Fetal surgery to cut off the blood supply to the parasite can greatly increase the chance of survival for the normal fetus, but carries some risk of miscarriage or other complications.

[edit] See also

Teratoma, including fetus in fetu

[edit] Fiction

Spoiler warning: Plot and/or ending details follow.

Conjoined twin myslexia (a fictional condition featured in an episode of South Park)
Brothers of the Head, a novel (and now a film) involving conjoined twin brothers with a parasitic triplet.
Jonas Venture Junior, A fictional parasitic twin (later separated) from the TV series The Venture Bros.
The Dark Half, a novel by Stephen King about an author with a pseudonym who begins physically manifesting himself and is ulitmately revealed to be a twin that was parasitically absorbed by the author in utero.
In Imprint (Masters of Horror episode), an American journalist meets a Japanese prostitute who is deformed by her parasitic twin which she hides beneath her hair. This parasitic twin resembles a hand with eyes and mouth, and can cause extreme pain to the prostitute when she doesn't get what she wants.