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. 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 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.
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Specific types of parasitic twin
Fetus in fetu
Fetus in fetu 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 until it grows so large that it starts to harm the host, at which point doctors usually intervene. Invariably the parasitic fetus is anencephalic (without a brain) and lacks internal organs, and as such is unable to survive on its own.
Acardiac twin
An acardiac twin is a parasitic twin that fails to develop a head, arms and a heart. The resulting torso can leech blood flow from the surviving normal twin, causing extreme stress on the normal fetus's heart.
Prenatal surgery must be performed if the normal fetus is to survive.
External Links
- Images of Acardiac twin (http://www.thefetus.net/page.php?id=1146) (note: contains anatomical pathology images)