TRAP syndrome, or twin reversed arterial perfusion sequence, is a rare complication of identical twin pregnancies in which one twin develops without a functioning heart and survives entirely by receiving blood from the other twin. It occurs in about 1% of monochorionic pregnancies (twins sharing a single placenta), which translates to roughly 1 in every 35,000 pregnancies overall. Without treatment, the mortality rate for the healthy twin exceeds 50%, but modern interventions have dramatically improved those odds.
How TRAP Syndrome Develops
In a normal twin pregnancy where both babies share a placenta, blood vessels on the placental surface can form connections between the two circulatory systems. In TRAP syndrome, these connections create a deeply abnormal situation: blood flows backward through one twin’s umbilical cord, effectively turning that twin into a passive recipient of already-used, oxygen-poor blood from the other twin.
The twin receiving this reversed blood flow, called the acardiac twin, never develops a working heart and often lacks a head, upper limbs, or most internal organs. Because the oxygen-depleted blood reaches the lower body first, the legs and lower trunk tend to be the only parts that grow at all, while the upper body remains severely underdeveloped or absent. This twin is not viable and cannot survive outside the womb under any circumstances.
The other twin, called the pump twin, is anatomically normal but faces a serious burden. Its heart must pump blood for two bodies instead of one. The larger the acardiac twin grows, the harder the pump twin’s heart has to work, creating an escalating cycle of cardiovascular strain.
Risks to the Healthy Twin
The central danger of TRAP syndrome is that the pump twin’s heart can fail under the extra workload. As the acardiac mass continues to grow, it draws more and more blood from the pump twin. This can lead to high-output heart failure, a condition where the heart simply cannot keep up with the demand being placed on it. Signs of this strain include fluid buildup around the pump twin’s heart and body, a condition known as hydrops fetalis.
Left untreated, more than 50% of pump twins die. The risk climbs as the acardiac twin grows larger relative to the pump twin. Preterm labor and excess amniotic fluid (from the pump twin’s overworked kidneys) are additional complications that threaten the pregnancy even when the pump twin’s heart is still coping.
How It’s Diagnosed
TRAP syndrome is typically discovered during a routine ultrasound when a sonographer notices that one twin in a shared-placenta pregnancy has no heartbeat yet continues to grow. Standard grayscale ultrasound can reveal the amorphous, partially formed acardiac twin and confirm the absence of cardiac activity. Cases have been identified as early as the first trimester, though some are not caught until later; one published case was discovered at 26 weeks.
The diagnosis is confirmed with Doppler ultrasound, which visualizes the direction of blood flow. In a normal fetus, blood flows away from the placenta through the umbilical vein and back toward it through the umbilical arteries. In the acardiac twin, this pattern is reversed: blood flows backward through the arteries, pushed in the wrong direction by the pump twin’s heart. Doppler imaging can also reveal the connecting blood vessels running across the placental surface between the two umbilical cords.
Beyond confirming the diagnosis, Doppler measurements help predict how the pump twin is doing. The difference in blood flow resistance between the two umbilical cords can serve as a prognostic marker. When that difference is large enough, the outlook for the pump twin tends to be more favorable.
Treatment Options
The goal of treatment is straightforward: cut off blood flow to the acardiac twin so the pump twin’s heart can recover. Several techniques exist, and all are performed while the pregnancy continues.
Radiofrequency ablation (RFA) is the most widely used approach. A needle-like device is inserted through the mother’s abdomen under ultrasound guidance, using local anesthesia for the mother. The needle is directed into the acardiac twin’s pelvis near the umbilical cord insertion, and radiofrequency energy is applied to coagulate the tissue, sealing off the blood vessels that feed the reversed circulation. The procedure has been used since the early 2000s and carries a pump twin survival rate of 85% to 88%. A Japanese study that followed 38 treated pregnancies long-term found zero cases of neurodevelopmental problems among surviving children, which is reassuring given the abnormal blood flow the pump twin endured before treatment.
Bipolar cord coagulation is another option, using electrical current delivered through a small instrument to seal the acardiac twin’s umbilical cord directly. A meta-analysis comparing techniques found survival rates of 86% for radiofrequency ablation, 82% for bipolar cord coagulation, 72% for laser cord coagulation, and 70% for cord ligation. The choice often depends on the specific anatomy of the pregnancy and the expertise available at a given center.
Monitoring During Pregnancy
Not every TRAP pregnancy requires immediate intervention. When the acardiac twin is small relative to the pump twin and the pump twin’s heart function looks stable, doctors may choose close surveillance instead. In these cases, ultrasound scans every two to three weeks track three things: the growth rate of the acardiac mass, the heart function of the pump twin, and amniotic fluid levels. Any sign that the pump twin’s heart is struggling or that the acardiac twin is growing rapidly can prompt a shift toward intervention.
After a procedure like radiofrequency ablation, a follow-up scan is typically done within one week to confirm the pump twin is alive and that blood flow to the acardiac twin has stopped. If both checks look good, the pregnancy returns to a standard monitoring schedule. Detailed fetal echocardiography, a specialized ultrasound focused on the pump twin’s heart, is part of the evaluation both before and after any intervention.
What the Acardiac Twin Looks Like
Because the acardiac twin receives only oxygen-poor blood, and because that blood preferentially supplies the lower body, the resulting anatomy can be striking and alarming on ultrasound. Some acardiac twins have recognizable legs and a partial trunk but no head, arms, or chest organs. Others are little more than an amorphous mass of tissue with some bone and skin. The umbilical cord connecting the acardiac twin to the placenta is often unusually short and may contain only a single artery instead of the normal two. None of these features are compatible with life, and families can be reassured that the acardiac twin has no capacity for consciousness or sensation.
Long-Term Outlook for the Pump Twin
When TRAP syndrome is caught in time and treated successfully, the pump twin generally does well. The 85% to 88% survival rate after radiofrequency ablation reflects outcomes through birth and the newborn period. Among pump twins who survive, the available long-term data is encouraging. The largest follow-up study to date found no increase in neurodevelopmental delays among children who underwent RFA in utero, suggesting that the period of abnormal blood flow before treatment does not cause lasting brain injury in most cases.
Prematurity remains the main risk factor for complications after birth, since some of these pregnancies deliver early either spontaneously or because of concerns about the pump twin. The earlier the intervention successfully stops blood flow to the acardiac twin, the more time the pump twin’s heart has to recover before delivery.