Parasitic twin

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A parasitic twin is the result of a situation related to the process that results in teratomas, 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 Fetus in fetu
- 1.2 Acardiac twin or TRAP sequence
2 See also
3 References
4 External links

[edit] Specific types of parasitic twin

[edit] Fetus in fetu

Fetus in fetu (or Fœtus in fœtu) describes an extremely rare abnormality that involves a fetus getting trapped inside of its twin. It continues to survive as a parasite even past birth by forming an umbilical cord-like structure that leeches its twin's blood supply until it grows so large that it starts to harm the host, at which point doctors usually intervene. [1] Invariably the parasitic fetus is anencephalic (without a brain) and lacks internal organs, and as such is unable to survive on its own, though it may have almost human (albeit underdeveloped and bizarre) features such as limbs, digits, hair, nails and teeth. Fetus in fetu is such a rare condition that only some 92 cases worldwide have ever been reported. Fetus in fetu happens very early in a twin pregnancy, when one fetus wraps around and envelops the other. The dominant fetus grows, while the fetus that would have been its twin lives on throughout the pregnancy, feeding off its host twin like a kind of parasite. Usually, both twins die before birth from the strain of sharing a placenta. Sometimes, however, the host twin survives and is delivered. The last case was reported in Temuco, Chile, in November, 2006.[2]

[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% or less 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

Craniopagus parasiticus
Conjoined twin myslexia (a fictional condition featured in an episode of South Park)
The Dark Half, a Stephen King novel where the main character is an example of fetus in fetu.
Alamjan Nematilaev, a boy who, aged 7, was found to host a parasitic twin, and whose case attracted worldwide media attention.
Brothers of the Head, a novel (and now a film) involving conjoined twin brothers with a parasitic triplet.

[edit] References

Hoefell et al. Fetus In Fetu: A Case Report and Literature Review. Pediatrics. 105 (6), June 2000, pp. 1335–1344.
ABC News A Pregnant Man?
MSNBC Chile Boy