Re: Fetus in fetu

It is possible this to occur and the term used to describe the condition 
is fetus in fitu.  It is a rare form of conjoined twinning and the 
term 'parasitic twin' is used.  


You may find the following website useful:
 http://www.answers.com/main/ntquery?

method=4&dsid=2222&dekey=Parasitic+twin&gwp=8&curtab=2222_1
Here is the text from that site incase the link becomes broken in the 
future:
parasitic twin 
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.


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.