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Twins / Multiples | Twin-reversed arterial perfusion (TRAP)

Twin-reversed arterial perfusion (TRAP) sequence (or Acardiac twin) is a very rare complication, seen in twins that share a single placenta (monochorionic twins). In TRAP sequence, one twin (called the “pump twin”) develops normally, but the other (called the “acardiac twin”) is very abnormal and has no chance of surviving outside the womb.

Essential Information

  • Twin-reversed arterial perfusion (TRAP) sequence is a very rare condition that only occurs in identical twins. This condition is also referred to as Acardiac Twinning.
  • In this condition, one of the fetuses, called the “acardiac twin”, does not have a normally-functioning heart and has no chance of surviving outside the womb.
  • The fetus with the normal heart function (called the “pump twin”) sends blood to the acardiac fetus, which causes it to grow. This places increased demand on the pump twin’s heart and may lead to heart failure or death.
  • The fetal intervention in severe cases of twin-reversed arterial perfusion includes closing off the blood source to the acardiac fetus, allowing the pump fetus to thrive.


Twin reversed arterial perfusion (TRAP) sequence is a complication specific to identical twins who share a single placenta (monochorionic twins). In this condition, one of the fetuses (referred to as the “acardiac twin”) does not have a normally-functioning heart, and as a result, is unable to survive after birth. During pregnancy, however, this acardiac twin receives its blood supply from the normal, ‘pump’ twin through blood vessel connections at the level of the placenta and through backward flow in the umbilical cord. As such, the acardiac fetus may grow before birth.

The acardiac fetus places an increased demand on the heart of the pump twin. If the acardiac fetus grows very large, this can cause heart failure and death in the pump twin.

TRAP sequence can be diagnosed by ultrasound. The acardiac twin may resemble an abnormal mass of tissue or a very abnormal, under-developed fetus with no heart beat. If heart failure is present in the pump twin, this may result in a condition called “hydrops”, which refers to abnormal fluid collections under the fetus’ skin, in the chest or in the abdomen, as well as excess amniotic fluid.

In cases where the acardiac twin remains small, and the pump twin does not develop heart failure, the outcomes for the pump twin can be excellent. However, when the acardiac twin becomes large, its blood flow must be interrupted so as to improve the survival chances of the pump twin.


No treatment is necessary in cases where the acardiac fetus remains small and heart failure does not develop in the pump twin. However, if the acardiac fetus grows large, and there is an excess of amniotic fluid and hydrops, fetal therapy is necessary to improve the survival chances of the pump fetus.

Prenatal therapy aims to stop the blood flow towards the acardiac fetus. This is done by closing off the umbilical cord of the acardiac mass (termed “cord occlusion”) or by closing off the large blood vessels within the acardiac fetus using either radiofrequency energy (termed “radiofrequency ablation (RFA)”), bipolar cord occlusion or interstitial laser. Outcomes are similar with all techniques, with survival of the pump twin in approximately 85% of cases. The average gestation at delivery after such procedures is 34-35 weeks’ of pregnancy. Developmental delay in survivors is rare, occurring in approximately 2 per cent of cases. If developmental delay occurs, it is most often a result of prematurity.

Referral Information


Monochorionic multiple pregnancy with twin reversed arterial perfusion sequence prior to 20 weeks gestation. Intervention after 22 weeks gestation will only be offered in the presence of signs of high-output cardiac failure in the pump-twin or a very large acardiac fetus.

Current Research Study

Patients seen before 14 weeks gestation will be offered participation in the TRAPIST-trial, which randomizes between early (12-14 weeks) and later (16-19 weeks) intervention in an attempt to prevent the 30% spontaneous loss rate that occurs between 12-16 weeks.