Twin-reversed arterial perfusion syndrome (TRAP or Acardiac twin)

Luís Flåvio de Andrade Gonçalves, MD1, Sandra Rejane Silva, MD2, Philippe Jeanty, MD, PhD3

1999-05-26-18 Twin-reversed arterial perfusion syndrome © Gonçalves

[1] Florianopolis, SC Brazil, [2] Sao Paulo — SP [3] Nashville, TN

Adapted and reproduced with permission from TKI Medcon Inc. http:/


Twin-reversed arterial perfusion (TRAP) sequence is a rare condition (0.3:10,000 births) that occurs in monochorionic twin pregnancies, resulting in coexistence of a normal "pump" twin and an acardiac twin.

Sonographic features

  • Pathognomonic finding: reversed arterial perfusion on Doppler. When imaging the umbilical cord with Doppler, arterial waveforms are observed from the placenta towards the acardiac twin. Venous blood flow takes the opposite direction. This finding results from the absence of a heart (pump) in the acardiac twin in association with artery-to-artery communications on the placenta, allowing the acardiac twin to get its blood supply from the normal twin.
  • Second abnormal fetus with impaired or absent development of:
  • cephalic pole
  • heart
  • upper limbs
  • viscera
  • Relative preservation of the lower limbs (although clubbing and abnormal toes are common)
  • 2-vessels cord
  • Inconsistent membrane development between the twins (from full sac to strips of membrane)
  • Occasionally the umbilical artery of the acardiac twin connects to the superior mesenteric artery (instead of the iliac artery). This is the persistence of an "old" viteline supply.


The pathognomonic finding of reversed arterial perfusion on Doppler. When imaging the umbilical cord with Doppler, arterial waveforms are observed from the placenta towards the acardiac twin: the pulsatile flow is directed down, towards the fetus. Venous blood flow takes the opposite direction.


Massive edema around the neck of the acardiac fetus.


Aspiration of fluid from a cystic collection of the neck of an acardiac fetus to ascertain its karyotype.


The ultrasound appearance and pathology specimen of the two cords (different cases). Notice their proximity, the 2-vessel and the smaller size cord of the acardiac twin.


The proposed pathogenesis is the association of paired artery-to-artery and vein-to-vein anastomoses through the placenta combined with delayed cardiac function of one of the twins early in pregnancy. This situation allows blood pumped from the healthy twin to perfuse retrogradely the heart of the other twin. Retrograde perfusion interferes with normal cardiac development, which rarely goes beyond the stage of tubular heart.


A late separation of the embryonic cell mass results in a monochorionic-diamniotic twin pregnancy. Anastomosis of vessels establishes a connection between the 2 circulations. Retrograde perfusion interferes with normal cardiac development, which rarely goes beyond the stage of tubular heart. Thus the acardiac fetus becomes dependent on the perfusion of the "pump" twin.


The upper half of the body of an acardiac twin is extremely poorly developed and, sometimes, not developed at all. Head, cervical spine and upper limbs are usually absent. Edema and sonolucent areas in the upper body, consistent with cystic hygroma, are common. In contrast, the lower half of the body, although malformed, is better developed. This pattern of development may be explained by the mechanism of perfusion of the acardiac twin. Blood that enters the abdomen of the fetus is deoxygenated blood that left the normal twin. Most of the oxygen available is extracted when the blood enters the acardiac twin, allowing for some development of the lower body and extremities. Once blood reaches the upper half of the body, oxygen saturation is extremely low, halting development of this area.

The acardiac twin is thus, a parasite. It requires blood pumped from the normal twin to keep developing, putting the pump fetus at risk of high output cardiac failure. The risk is directly dependent on the size of the acardiac twin: the higher the weight of the acardiac twin, the higher the risk of cardiac failure and death for the normal twin. Overall only 50% of pump twins survive.


Not uncommonly, the umbilical vein of acardiac fetuses is replaced to the iliac vein as in this case where I injected contrast medium in the umbilical vein.


Management has consisted in termination of pregnancy or interruption of flow to the acardiac fetus, by surgical extraction and ligation, fetoscopic umbilical cord ligation, embolization, laser vaporization. A conservative management aimed at supporting the cardiac function of the pump twin with Digoxin has also been attempted. Large numbers are not available to compare the various techniques.


Authors have had a field day at creating a nomenclature for each and every variant (table). This is of little significance, since the disorder is invariably fatal, and the only concern is the preservation of the pump twin.

Name Malformation
Acephalus No cephalic structure present
Anceps Some cranial structure and or neural tissue present
Acormus Cephalic structure but no truncal structures
Amorphus No distinguishable rostral or caudal structure



Several acardiac fetuses: note the caudo-rostral development gradient with poor formation of the cephalic end.




Several hands with various degrees of oligodactyly.


Astomia (absence of the mouth)

Differential diagnosis

Few entities can resemble an acardiac twin. Occasionally a twin to twin transfusion could impose for a TRAP. These can be differentiated by the recognition of a membrane (even in a stuck twin), and of course, cardiac activity in the smaller fetus. A fetal demise in a twin pregnancy could also look like an acardiac, however, there should be no Doppler signal in the dead fetus.

Associated syndromes



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