Obstetrics and Gynecology

TRAP Syndrome

Twin Reversed Arterial Perfusion (TRAP) Syndrome or Sequence

1. What every clinician should know

Clinical features and incidence

Twin reversed arterial perfusion (TRAP) sequence is a rare condition unique to identical twins. Overall, TRAP affects about 1% of all monozygotic twins with 75% occurring in monochorionic-diamniotic twins and the remaining 25% in monoamniotic twins. TRAP is characterized by a normal appearing pump twin that sustains a dysmorphic acardiac twin through aberrant arterioarterial anastomoses within the shared single placenta.

Risk factors

Only monozygotic twins with monochorionic-diamniotic or monoamniotic placentation are at risk for TRAP. In vitro fertilization increases monozygotic twinning so early screening for multiple gestations with determination of chorionicity if present is warranted. All monochorionic twins, conceived spontaneously or with assisted reproduction, should be followed with serial ultrasounds to ensure normal development of both twins.

2. Diagnosis and differential diagnosis

A. Establishing the diagnosis

The diagnosis of TRAP requires monochorionic placentation, a normal appearing pump twin, an abnormal appearing twin without discernible cardiac activity and paradoxical arterial flow towards, rather than away from, the acardiac twin. It is this relatively deoxygenated arterial flow from the pump twin to its co-twin through abnormal placental anastomoses that is theorized to contribute to the abnormal development of the acardiac twin.

The acardiac twin often lacks a recognizable head, trunk, and upper extremities but has demonstrable flow on color Doppler studies. (Figure 1) (Figure 2) The pump twin usually has normal fetal anatomy but is at risk of hemodynamic compromise as it is responsible for supporting the circulation of its co-twin as well as its own. The risk of high-output cardiac failure in the pump twin is related to the size of the acardiac twin and often determines whether in utero treatment is recommended.

Figure 1.

Abnormal acardiac twin adjacent to head of normal pump twin.

Figure 2.

Color flow imaging shows flow in the umbilical cord (arrow) but no demonstable flow in the fetal chest in the region of the heart.

A careful search for associated structural malformations is indicated in cases of TRAP. Abnormalities of the umbilical cords are quite common in both twins, with a single umbilical artery being present in up to two-thirds of cases. (Figure 3) In addition to serial sonograms and color Doppler studies to establish the diagnosis of TRAP, fetal DNA analysis and fetal echocardiography are also recommended.

Figure 3.

Two vessel umbilical cord, as demonstrated by color Doppler, is a common finding in TRAP sequence.

Up to 10% of TRAP will have associated chromosomal abnormalities and 5-10% will have other structural defects, with congenital heart disease being most common. Fetal echocardiography is also useful to assess cardiac function of the pump twin and need for intervention. Fetal MRI may be useful if there is a suspicion of associated anomalies, particularly those involving the brain of the pump twin.

B. Differential diagnosis

TRAP is often overlooked as a possible diagnosis when one twin of an identical pair is noted not to have visible cardiac activity early in gestation. However, rather than early single twin demise, it becomes clear on subsequent imaging that both twins are continuing to grow despite the absence of cardiac activity in one of the twins. Color flow mapping can help to distinguish TRAP from single twin demise as there will be absent flow in the setting of intrauterine fetal death compared to demonstrable flow in an acardiac twin.

In very rare circumstances, a placental mass such as a teratoma, may have sonographic features that resemble an amorphous acardiac twin. Both of these will have discernible flow on color Doppler imaging but only the acardiac twin will have an umbilical cord.

3. Management


Confirmation of TRAP is often delayed due to an earlier diagnosis of a single twin demise in a monochorionic pregnancy. However, once the diagnosis is established, genetic counseling should be offered along with fetal DNA analysis. A comprehensive anatomy survey of the pump twin should be completed along with fetal echocardiography and perhaps fetal MRI in selected cases.

Pregnancy options, including termination, should be discussed as even with normal fetal DNA sequencing and normal fetal anatomy, the pump twin remains at risk of high-output cardiac failure, preterm delivery and perinatal death. Adverse perinatal outcomes are even greater in cases with monoamniotic placentation, in which cord entanglement and subsequent neurologic injury or death can occur.

For ongoing pregnancies, the need for in utero treatment to optimize the outcome for the normal pump twin must be assessed. The risk of hemodynamic compromise of the pump twin is greatest when the acardiac twin is large. A frequently used formula to estimate the weight of the acardiac twin in grams is (1.2 x length 2) – (1.66 x length), where length is the longest linear measurement in cm. Using estimated fetal weights, an acardiac-to-pump ratio of greater than 0.7 is associated with congestive heart failure in the pump twin in about a third of cases. When the acardiac twin remains small relative to the pump twin, a more favorable outcome is expected.

Selective reduction of the acardiac fetus is favored in cases of previable TRAP when the acardiac-to-pump ratio is greater than 0.5, the acardiac twin is rapidly increasing is size, or the pump twin shows evidence of cardiac dysfunction. (Figure 4) Although selective reduction by cord occlusion can accomplished by a number of different techniques, radiofrequency ablation (RFA) using an ultrasound-guided, percutaneous approach is preferred by many fetal interventionalists due to its minimally invasive nature and favorable outcomes. Under ultrasound guidance, the RFA needle is inserted into the intrafetal cord insertion site of the acardiac twin and then the application of radiofrequency immediately interrupts blood flow to the acardiac twin. (Figure 5)

Figure 4.

Pericardial effusion and cardiomegaly (arrow) in the presence of polyhydramnios indicates advanced high-output cardiac failure in the pump twin.

Figure 5.

Selective reduction of an acardiac twin via ultrasound-guided, percutaneous radiofrequency ablation (RFA).


Untreated TRAP cases in which the acardiac twin is small and the pump twin has no demonstrable hemodynamic compromise can be managed similarly to other identical twins based on placentation. In general, monochorionic-diamniotic twins are delivered at 36 weeks and monoamniotic twins at 34 weeks provided the growth and antenatal testing remains reassuring. A trial of labor and vaginal birth are possible in monochorionic-diamniotic twins if the pump twin is presenting and in vertex presentation, reserving cesarean delivery for standard obstetric indications.

While cesarean delivery is a common practice for monoamniotic twins, a trial of labor may be considered in TRAP if the same conditions are met. Earlier delivery may be necessary if the pump twin shows evidence of cardiac dysfunction beyond the therapeutic window for RFA. In these cases, antenatal steroids are administered if time permits and cesarean delivery is usually recommended.

Following successful cord occlusion of an acardiac twin, patients with a healthy pump twin can be managed similarly to a singleton. If fetal growth is acceptable and fetal testing is reassuring, it is reasonable to await spontaneous labor if the patient is a candidate for a vaginal delivery. In both treated and untreated cases of TRAP, it is important that all products of conception are delivered and the uterus is empty after completion of the third stage of labor.


In the majority of cases, a normal postpartum course can be expected. Retained products of conception resulting in postpartum hemorrhage is a potential complication in patients who have undergone selective reduction of the acardiac twin. Preterm delivery, problems related to prematurity, prolonged hospital stay or residual cardiac dysfunction in the pump twin may increase the risk of postpartum depression. However, most patients have a good understanding of TRAP sequence by the time of delivery and are prepared for possible complications.

4. Complications

Complications as a consequence of the condition

The major risk of the TRAP sequence is the development of high-output cardiac failure in the pump twin and its associated consequences. Without intervention, congestive heart failure is reported to occur in about 30% of pump twins. Serial sonographic surveillance and fetal echocardiography are warranted to monitor heart function which if deteriorating may be an indication for in utero therapy or early delivery, depending on gestational age. Hydrops or death of the pump twin may result in such cases if no action is taken. This occurs in about 25% of untreated TRAP cases.

Heart failure in the pump twin can also be accompanied by polyhydramnios in about 50% of cases, placing the patient at increased risk for cervical shortening, premature rupture of membranes, and preterm labor and delivery. While cardiac dysfunction may persist for some time in an affected pump twin, both in utero after selective reduction of the acardiac twin and after birth, the greatest threat to the surviving pump twin is prematurity and its complications.

Complications as a consequence of management

Cord occlusion by any method may be difficult to accomplish, incomplete or unsuccessful. This may result in hemodynamic disturbances that might adversely affect the pump twin, resulting in neurologic damage or fetal death. RFA has an overall complication rate of about 5% which includes preterm labor, preterm premature rupture of membranes, fetal or maternal bleeding, infection and maternal thermal injuries.

Careful sonographic mapping of maternal structures as well as the twins, the placenta, and cord insertion locations can help to identify a safe entry site, thereby reducing some of these risks. Pre and post-procedural antibiotics can reduce the risk of infection and proper placement of appropriate grounding pads can eliminate the risk of thermal burns. A short course of tocolysis may also be considered in a patient who demonstrates uterine irritability before or following the procedure.

5. Prognosis and outcome

Fetal/neonatal outcomes

Without intervention, the perinatal mortality rate in TRAP pregnancies is 35-55%. In contrast, selective reduction of the acardiac twin via RFA is associated with survival of the pump twin in over 90% of TRAP cases. Data on long-term outcomes of pump survivors is limited. Given that there is potential for hemodynamic fluctuations in TRAP and with selective reduction in a monochorionic gestation, evaluation for possible neurologic injury and monitoring the child’s mental and motor development is warranted.

Long term impact

In general, a TRAP pregnancy has no lasting effects on a patient’s well-being apart from the possible need for a cesarean delivery and its potential impact on future health. If the pump twin has complications of prematurity, chronic cardiac dysfunction or fails to survive, there may be psychological consequences that may impact the mother’s long-term mental health. Overall, favorable outcomes are expected for patients with TRAP pregnancies.

6. What is the evidence for specific management and treatment recommendations

Chalouhi, GE, Stirnemann, JJ, Salomon, LJ, Essaoui, M, Quibel, T. "Specific complications of monochorionic twin pregnancies: twin-twin transfusion syndrome and twin reversed arterial perfusion sequence". Seminars in Fetal and Neonatal Medicine. vol. 15. 2010. pp. 349-56.

(Good review of TRAP.)

Jelin, E, Hirose, S, Rand, L, Curran, P, Feldstein, V. "Perinatal outcome of conservative management versus fetal intervention for twin reversed arterial perfusion sequence with a small acardiac twin". Fetal Diagnosis and Therapy. vol. 27. 2010. pp. 138-41.

(Role for expectant management when acardiac twin is small.)

Lee, H, Wagner, AJ, Sy, E, Ball, R, Feldstein, VA. "Efficacy of radiofrequency ablation for twin-reversed arterial perfusion sequence". American Journal of Obstetrics and Gynecology. vol. 196. 2007. pp. 451-58.

(Large series of TRAP treated with RFA from single center.)

Livingston, JC, Lim, FY, Polzin, W, Mason, J, Crombleholme, TM. "Intrafetal radiofrequency ablation for twin reversed arterial perfusion (TRAP): a single-center experience". American Journal of Obstetrics and Gynecology. vol. 197. 2007. pp. 399.e1-339.e3.

(Second largest series of RFA treated TRAP from single center.)

Spadola, AC, Simpson, LL. "Selective termination procedures in monochorionic pregnancies". Seminars in Perinatology. vol. 29. 2005. pp. 330-7.

(Review of different techniques for cord occlusion in identical twins.)
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