The undeveloped twin is termed as parasitic, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.

Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus' heart.

[3] A rare variant of the acardiac fetus is the acardius acormus where the head is well developed but the heart and the rest of the body are rudimentary.

While it is thought that the classical T.R.A.P./Acardius sequence is due to a retrograde flow from the umbilical arteries of the pump twin to the iliac arteries of the acardiac twin resulting in preferential caudal perfusion, acardius acormus is thought to be a result of an early embryopathy.

[5] Fetus in fetu sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity.