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Hubaishi, Cherifi, Elsayed, and Adam: Twin reversed arterial perfusion (TRAP) sequence – a case report and review of literature


Twin reversed arterial perfusion (TRAP) sequence (acardiac monster, chorioangiopagus parasiticus) is a rare complication of monochorionic twin pregnancy, occurring at an overall incidence of 1 in 35 000 deliveries, or 1% of monochorionic twin pregnancies.1,2

Twin reversed arterial perfusion sequence is characterized by a structurally normal pump twin perfusing an anomalous recipient twin via an artery-to-artery anastomosis in a reversed direction.3,4 The perfused twin is gradually transformed into a parasite-like organism dependent on the blood supply from the pump twin. The deoxygenated blood leaving the pump twin through the umbilical artery preferentially perfuses the lower extremities of the perfused twin through the umbilical artery anastomoses. This abnormal pattern of perfusion is thought to be the cause of the predictable arrangement of multiple anomalies seen in these fetuses, which are more pronounced in the superior aspect of the body and head and usually accompanied by normal lower extremities.3,5 TRAP sequence pregnancies are threatened by several obstetric complications, such as hydrops fetalis, congestive heart failure, polyhydramnios, umbilical cord accidents, preterm delivery and/or fetal death of the pump twin.6 Poor fetal outcome for the normal (pump) twin has been associated, in the literature, with a large twin weight ratio (TWR), and this finding is purported to be an indication for feto-occlusive treatment of the perfused twin. Here we present a case of acardiac twin pregnancy with a TWR of around 44%, complicated by polyhydramnios but without any sign of cardiac failure, managed expectantly, with good neonatal outcome for the pump twin.

Case report

The patient is a 30-year-old, gravida 2, para 1 woman, in a second-degree consanguineous marriage. An ultrasound was performed by her primary physician at 18 weeks and she was given a diagnosis of twin gestation with intrauterine death of one twin. Subsequent ultrasound revealed that the ‘dead’ twin was growing.

She was referred to our antenatal clinic in Dubai Hospital, Dubai, at 30 + 4 weeks’ gestation for further management. She denied any medical disease. Her first pregnancy was uneventful; she delivered at term a healthy male newborn in 2010. The index pregnancy was spontaneous with no history of early pregnancy complications. Her physical examination was within normal limits. The obstetrical examination revealed a large-for-date uterine size, palpable fetal movements and only one audible fetal heart.

A detailed ultrasound performed in our department revealed a twin gestation with the following findings:

  • The pump twin was in a cephalic position with biometric parameters consistent with known gestational age.

  • Fetal anatomy was structurally normal; the fetal cardiac anatomy was also normal without signs of cardiomegaly, cardiac failure or pulsatile venous flow in the pump twin. Interestingly, this fetus demonstrated unusual posturing with a continuously extended head and all four limbs flexed throughout the examination.

  • Fetal movements appeared normal and there was no evidence of mass seen at the front or behind the neck to explain the abnormal posturing of the head. The spine was normal.

  • Doppler examination showed normal umbilical waveforms (Figure 1). The middle cerebral artery peak systolic velocity was not suggestive of severe fetal anaemia. Four-chamber view of the pump twin was normal (Figure 2). There was no cardiomegaly or pericardial effusion.


Normal umbilical artery flow.


Normal aspect of four-chamber view.


The recipient twin demonstrated markedly abnormal features and was amorphous. There was significant subcutaneous oedema, with no evidence of cephalic pole or superior limbs; primitive thorax without pulmonary tissue or demonstrable cardiac chambers. Inferior limbs were absent, except a single short femur. The spinal column was present in the dorsal and lumbar segments only (Figure 3). No identifiable abdominal structures were present (Figure 4). The length of the amorphous mass was approximately 23 cm.


Acardiac twin, rudimentary spine.


Abdominal circumferences of the pump twin and acardiac twin with no identifiable organs.


The umbilical cord contained a single umbilical artery. Colour Doppler imaging revealed a short umbilical cord between the recipient and marginal placenta (Figure 5).


Anastomosis to the acardiac twin on colour Doppler imaging.


Only one placenta was present and was located at the posterior wall with no visible dividing membrane. Polyhydramnios was present with a maximum vertical fluid pocket of 10 cm. These ultrasound findings were consistent with a diagnosis of monochorionic, monoamniotic twin with TRAP.

After extensive counselling and discussion with the patient, conservative management was continued with weekly ultrasound to detect any congestive cardiac failure of the pump twin. Corticosteroid therapy was initiated to optimize the fetal lungs in anticipation of preterm delivery. Amnioreduction was not indicated as the patient was not symptomatic. Within 2 days of arrival at our department, the patient complained of absence of fetal movement for more than 3 hours. Cardiotocography demonstrated a normal baseline fetal heart rate (128–130 beats per minute) with poor variability (less than 5 beats per minute) and no accelerations within 60 minutes. Regular contractions were recorded. The fetal biophysical profile was worrisome with no fetal movements, no tone and no demonstrable breathing movements seen with 30 minutes of scanning. An emergency caesarean section was performed. The pump twin was delivered in good condition, weighing 1620 g with Apgar scores of 7 and 8 on the first and fifth minutes, respectively. Umbilical cord arterial gas showed a pH of 7.37. No gross anomalies were found. Cardiac failure was not encountered in the 10 days the infant was in the neonatal intensive care unit. The neck and the limbs were normal (Figure 6).


Normal pump twin.


The acardiac twin weighed 1480 g and was clearly amorphous (Figure 7). The examination showed an arterial to arterial anastomosi. Fetal hair was seen at the cephalic pole and upper and lower segments were absent, except a small right lower limb bud, which was identifiable but abnormally placed. The umbilical cord was inserted in the upper part of the body with a single umbilical artery. An artery-to-artery anastomosis was identified after delivery and monoamnionicity was confirmed. The placenta was sent for examination. The report was in favour of clinical findings: central cord with three vessels, velamentous cord with two vessels, monochorionic monoamniotic gestation.


Acardiac twin (amorphous).



Twin reversed arterial perfusion sequence, or acardiac twin, is a highly morbid complication of monochorionic twinning. Aberrant arterial perfusion of one fetus, with deoxygenated blood from the other through direct arterial anastomoses, is responsible for the spectrum of morphological abnormalities seen in the perfused twin, as in our index case. Diagnosis was made by the absence of identifiable cardiac pulsation and poor definition of the head, trunk and upper extremities, deformed lower extremities and marked tissue oedema and abnormal cystic areas in the upper part of the body of the affected twin.3,7 Pulsed and colour Doppler imaging showed a reversed arterial flow pattern coming from the pump twin towards the parasitic twin, with blood supply entering the acardiac twin via a single umbilical artery.

Although this condition is diagnosable in the first trimester, optimal management is controversial. The natural history of this condition often manifests as development of cardiac failure, polyhydramnios, fetal hydrops and/or intrauterine death in the normal fetus, with mortality rates from 50% to 75% in the healthy pump twin, as reported in the literature.2 Prognosis in the perfused twin is uniformly fatal. Signs of worsening prognosis can include accelerated growth of the perfused (acardiac) fetus relative to its normal co-twin such that the abdominal circumference or fetal size of the acardiac fetus exceeds that of the normal twin. Prognosticating outcome in this condition has been aided by calculation of the TWR (estimated weight of the acardiac twin/estimated weight of the normal twin) using the equation: weight (g) = 1.2L2 − 1.7L (L being the largest diameter in centimetres of the acardiac mass). A TWR of > 50% has been found to increase predicting preterm delivery, intrauterine death and cardiac failure in the pump twin, whereas risk for the fetus with a TWR of < 50% is thought to be negligible.2

Many articles have been published outlining operative manoeuvres to interrupt blood flow to the perfused twin and limit consequences for the pump twin in whom mortality may be as high as 50% secondary to cardiac decompensation or preterm delivery. Radiofrequency ablation, fetoscopic selective ligation of the acardiac cord and fetoscopic and/or sonographic embolization or ablation of the umbilical vessels using alcohol, platinum coils or other thrombogenic material has been reported8 to manage this condition. Current indications for optimal timing of these interventions have been primarily based on relative weights of the perfused (abnormal) fetus to the normal (pump) co-twin with intervention suggested when the TWR exceeds 50%. We opted to manage expectantly and not to intervene in this case as the TWR was estimated to be 44%, and functionally the pump twin did not manifest signs of impending heart failure. This attitude was also adopted by others.5,6 In fact, despite availability of invasive fetal therapy for TRAP sequence, several articles have concluded that conservative or expectant management is reasonable for the subset of affected twins with TWR ≤ 50%. Sullivan et al.,9 in their study of 10 cases of TRAP managed expectantly, reported nine women who delivered healthy pump twins with one neonatal death. The mean gestational age at delivery was 34.2 weeks.9 The decision to manage expectantly was again based on the calculated weight ratio of the acardiac (perfused) twin to the normal (pump) twin. More recently, Jelin et al.5 reported the outcome of 18 TRAP twins managed conservatively and demonstrated pump twin survival rates of 88–91% in non-hydropic twins. The average age at delivery of twins in this cohort was 35 weeks.5

In our index case, the patient presented at 30 + 4 weeks, when the diagnosis of TRAP sequence was made, with a TWR of 44%. Expectant management was elected as cardiac decompensation was not evident. Preterm delivery occurred at 30 + 6 weeks because of fetal health concerns and poor biophysical parameters; however, cardiac failure was not encountered. This suggests that factors other than TWR must play a part in the development of haemodynamic compromise in these fetuses.

In 2012, Hartge and Weichert7 reported that, despite the risk of adverse pregnancy outcome of the pump twin, early diagnosis and intensive fetal surveillance in this condition can result in good fetal outcome in twin pairs with both high and low TWR. In the series of cases reported by Hartge and Weichert7 from a single tertiary care centre, diagnoses were made at a mean gestational age of 20.4 weeks (13.1–28.0) weeks and delivery occurred beyond completion of 31 gestational weeks in all cases. Delivery was predominantly by caesarean section because of preterm labour and/or rupture of membranes in TRAP sequence pregnancies with secondary polyhydramnios, or early signs of cardiac decompensation in the pump twin.7,8 Survivors were followed for a minimum of 6 months without evidence of adverse outcome. These outcomes mirror the progress of pregnancy described in most studies. In our case, even if the TWR did not reach the cut-off of 50%, it is still high; however, the good fetal outcome and third trimester delivery may lead us later to manage the pump twin expectantly as long as there are no signs of cardiac failure.

Although the exact causes of progressive decompensation of a pump twin are unclear, the recent literature suggests that cardiac decompensation is not inevitable nor is the need for invasive fetal therapy uniform in these cases, despite a high TWR. Close serial surveillance of the fetal cardiac function is, however, warranted to guide appropriate management. The appropriate timing of delivery in TRAP pregnancies, without evidence of cardiac compromise in the pump twin, remains uncertain. Conservative management of TRAP syndrome, consisting of serial ultrasound to assess the growth rate and cardiovascular status of the normal twin, can be undertaken with consideration of early delivery because of the high risk of sudden intrauterine fetal demise.


Twin reversed arterial perfusion is a rare complication of twin pregnancy. Diagnosis can be carried out from the first trimester and ultrasound with Doppler assessment will assess the growth of the acardiac twin and signs of cardiac decompensation in the pump twin.

Although there is not yet clear consensus in the management of TRAP, the most recent publications recommend following TRAP by weekly ultrasound to detect any deterioration in the pump twin. The decision of whether or not to use invasive treatment should be based on Doppler flow pattern and perfused/pump twin ratio.



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Roets E, Havenith MG, Klumper FJ, Arabin B, van Eyck J. Twin reversed arterial perfusion sequence. Eur Clinics Obstet Gynaecol 2006; 2:18–23.


Steffensen TS, Gilbert-Barness E, Spellacy W, Quintero RA. Placental pathology in TRAP sequence: clinical and pathogenetic implications. Fetal Pediatr Pathol 2008; 27:13–29.


Guimaraes CV, Kline-Fath BM, Linam LE, Garcia MA, Rubio EI, Lim FY. MRI findings in multifetal pregnancies complicated by twin reversed arterial perfusion sequence (TRAP). Pediatr Radiol 2011; 41:694–701.


Jelin E, Hirose S, Rand L, et al. Perinatal outcome of conservative management versus fetal intervention for twin reversed arterial perfusion sequence with a small acardiac twin. Fetal Diagn Ther 2010; 27:138–41.


Erol AO, Altay MM, Kaplan M, Kocak A, Karadeniz RS, Gelisen O. Good perinatal outcome of the pump twin in the twin reversed-arterial-perfusion sequence diagnosed at the ninth week of pregnancy and managed conservatively. J Matern Fetal Neonatal Med 2009; 22:952–5.


Hartge DR, Weichert J. Prenatal diagnosis and outcome of multiple pregnancies with reversed arterial perfusion (TRAP-sequence). Arch Gynecol Obstet 2012; 286:81–8.


Guimarães Filho HA, da Costa Lavoisier Linhares D, Araujo Júnior E, et al. Treatment of twin reversed arterial perfusion sequence by septostomy and amniodrainage: a case report. Arch Gynecol Obstet 2007; 275:489–93.


Sullivan AE, Varner MW, Ball RH, Jackson M, Silver RM. The management of acardiac twins: a conservative approach. Am J Obstet Gynecol 2003; 189:1310–13.

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