Twin reversed arterial perfusion (TRAP sequence) is a rare condition of monochorionic twin pregnancies. It arises when the cardiac system of one twin does the work of supplying blood for both twins. The twin supplying the blood is known as the "pump twin" and develops normally in the womb. However, the increased pumping of the heart puts this twin at risk for cardiac failure. The other twin — known as the "acardiac twin" — lacks a heart or has one that is not fully formed. It usually has a poorly developed body and may also be missing a head, limbs and torso.

With TRAP, it is believed that very early in pregnancy, a dramatic, ongoing fluctuation of blood pressure and blood flow may interfere with the development of the heart in the acardiac twin. Though the pump twin’s heart may go on to develop properly, the vessel connections between the twins can create problems. In cases of TRAP, the blood from the healthy pump twin flows backwards from the placenta into the unhealthy acardiac twin. Because the acardiac twin lacks a functional heart, it is unable to reciprocate and provide the forward flow of blood found in a normal fetal circulatory system.

Because the acardiac twin fails to fully develop vital organs, survival outside of the womb is not possible. In fact, the acardiac twin is technically not a fetus, but a mass of tissue that grows because of the blood flowing from the pump twin. Fetal interventions focus on monitoring the heart health of the pump twin and include procedures to stop blood flow to the acardiac twin.

Radiofrequency ablation (RFA) may be recommended in cases where the gestational age is greater than 16 weeks. This procedure involves the insertion of a small needle into the uterus. The needle contains several sharp prongs that disperse heat energy of radio frequency. The heat energy is used to seal the feeding vessels supplying blood to the acardiac twin at the base of its umbilical cord.

Any prenatal fetal procedure presents risks to both mom and baby. Following cord coagulation, the portion of the placenta no longer receiving blood supply from the circulation of the demised twin undergoes degenerative changes that increase the risk of preterm labor.