Parasitic or Heteropagus Twins are asymmetric conjoined identical or monozygotic twins in which the tissues of a severely defective twin (the parasite) are dependent on the cardiovascular system of the other, largely intact twin (autosite) for survival. The condition only occurs when an undeveloped or underdeveloped twin is attached to parts of the body of the twin that develops and is birthed.

The estimated incidence of of this syndrome is approximately 1 per 1 million live births. Isolated case reports comprise most of published work on this rare congenital anomaly.

The parasitic twin syndrome has been noted in dozens of official cases, but is thought to occur more often due to lack of official documentation of cases in third world countries. Many of these survived, often after surgery to remove the diseased twin.

One case was reported in 1783. The boy did not die from the cranio-defect but from a snake bite at the age of 4. In 2003, a girl was born with this syndome and lived a little more than a year, dying after an 11 hour procedure to remove the parasitic twin. Another living case occurred in 2005 in Egypt. That baby died after surgery to remove the diseased twin as well.

A-ke was born in the district of Yun-Ping Heen in China, with another male child of nearly the same size united to the pit of the stomach by the neck, as if his brother had plunged its head into his breast. 

It seems, from the noted cases, that the only case of parasitic twin to survive was associated with leaving the extra twin head in place instead of removing the head. Generally, the outcome in all cases of is death.

Some cases are associated with a living parasitic twin. In the case of the Egyptian baby, the external head was able to blink, smile and cry. There was no body associated with that twin. The developed baby did not die from the removal of the other twin, but rather from an infection associated with the surgery.

The parasitic twin syndrome is also referred to as the "two-headed" syndrome and "conjoined twin" syndrome. The conjoined twin syndrome is not the same, but many people interchange the two terms. The parasitic twin syndrome is more associated with a twin that is unable to live without being attached to the head of the living twin.

The vanishing twin syndrome is closely related. When vanishing twin syndrome occurs, the twin that dies in utero is absorbed by the living fetus or the mother. In most cases, the twin that is absorbed disappears completely with no adverse effects on the living baby. While there is no known cause for the vanishing twin syndrome or the subsequent parasitic twin syndrome, it is thought that the vanishing twin was developing with a placenta that did not provide enough support for the fetus.

IVF, or in vitro fertilization, is associated with higher than normal numbers of vanishing twins. This is due to the number of eggs implanted and the higher than normal rates of multiples. 

Many medical procedures associated with parasitic twins are experimental due to the paucity of cases.