Twin reversed arterial perfusion

Last revised by Joshua Yap on 03 Aug 2022

Twin reversed arterial perfusion (TRAP) sequence is a rare complication of monochorionic pregnancies. It develops when the following conditions are present:

1. lack of a well-formed heart in one of the twins (so-called acardiac twin), and

2. a superficial artery to artery placental anastomosis providing perfusion of the acardiac twin by the donor (pump) twin.  This can be shown on Doppler ultrasound by reversal of the acardiac twin umbilical arterial blood (i.e. flow towards the fetus).

It is thought to occur in ~0.5% (range 0.3-1% 1,4) of monozygotic pregnancies with an estimated incidence of 1:35,000 births overall 1,10

It can rarely occur with higher-order multifetal pregnancies 3.

TRAP is considered to primarily result from an abnormal placental arterial-to-arterial anastomosis. Venovenous anastomoses may also be present 11.

Classically this results in one normal and one abnormal twin:

The acardiac twin is haemodynamically disadvantaged receiving deoxygenated blood from donor twin which then supplies iliac arteries first. This will result in more perfusion compromise in the upper part of the body so atrophy of the heart and dependent organs (brain) are more pronounced. Eventually, a characteristic set of anomalies including acardia and acephalus develop. 

Acardiac twin

  • severely abnormal, with variable degrees of morphologic development (see morphologic types above)
  • the heart is absent or structurally abnormal, with <20% of fetuses having identifiable cardiac tissue 15
  • most commonly the head and upper extremities are absent or severely underdeveloped, while the lower body is better developed
  • may be amorphous with no distinguishable anatomic structures
  • severe subcutaneous edema and cystic hygromas common
  • single umbilical artery common

Pump twin

  • often, but not invariably, is structurally normal
  • multiple anomalies have been described, including at risk high output cardiac failure


  • on Doppler interrogation, flow in the acardiac twin umbilical arteries seen entering the fetus and blood leaving via the umbilical vein 13

Fetal MRI may be considered as an adjunct to ultrasound, to evaluate non-cardiac related abnormalities in the pump twin 15

TRAP sequence pump twin is at risk of:

  • cardiac failure
  • cerebral ischemic sequelae
  • preterm birth
  • fetal demise in utero

The risk of adverse outcomes in the pump twin is closely related to its size ratio to the acardiac twin.

As the acardiac twin is non-viable, the majority of efforts in management are focused on maintaining the viability of the other donor (pump) twin. The perinatal mortality for the pump twin can be as high as 50% 10.

Treatment is around the surgical destruction of the inter-twin anastomosis and includes:

  • endoscopic laser coagulation/radio-frequency ablation 5
  • surgical (fetoscopic) ligation of acardiac twin umbilical cord
  • selective delivery of acardiac twin 7

ADVERTISEMENT: Supporters see fewer/no ads

Cases and figures

  • Figure 1
    Drag here to reorder.
  • Case 1
    Drag here to reorder.