Twin reversed arterial perfusion sequence (TRAP sequence), also known as acardiac twinning, is a rare condition that occurs in pregnancies of identical twins that share one placenta. Twins that share a placenta are known as monochorionic twins.

TRAP sequence occurs in 1 percent of monochorionic twin pregnancies and 1 in 35,000 pregnancies overall. In TRAP sequence, one twin is developmentally normal (pump twin) while the other develops without a functioning heart (acardiac twin) and many other structures (head, limbs, etc.) that would allow this twin to develop into a normal fetus and newborn. The two are joined by a large blood vessel between their umbilical cords that passes through the placenta. The acardiac twin receives all of its blood from the pump twin. This causes the pump twin's heart to work harder than it typically would in a normal pregnancy.

TRAP sequence is a condition where blood flow to and from the acardiac twin is opposite of the way blood is normally supplied. In a normal pregnancy, low oxygen blood is pumped from the fetus to the placenta where it gets oxygen from the mother and is then sent back to the fetus through the umbilical cord.In TRAP sequence, the healthy twin pumps blood through the placenta to the acardiac twin in the opposite direction of the normal pumping system. The hardworking heart of the pump twin keeps both twins alive but puts the pump twin at risk for heart failure and death if left untreated. The larger the acardiac twin gets, the harder the pump twin works, increasing the risk of heart failure.

TRAP sequence is detected during a routine prenatal ultrasound, which shows a single placenta containing two fetuses. It may also show excess amniotic fluid surrounding the twins, who often share the same amniotic sac. In these cases, obstetricians will likely refer the mother to a maternal-fetal medicine specialist also know as a perinatologist, who specializes in high-risk pregnancies. The physician will confirm the diagnosis through the use of fetal echocardiography, a specialized ultrasound of the heart and blood vessels that shows the flow of blood between the twins and confirms that TRAP sequence is taking place.

With proper care, 80 to 85 percent of pump twins survive and go on to live a healthy life. The acardiac twin does not develop a heart structure and will not survive after birth.

The goal of prenatal care is to minimize the risk of heart failure for the pump twin. In heart failure, the amount of blood pumped by the heart is not enough to meet the body's needs. In this case, the needs are double as the healthy twin pumps blood for both fetuses.

When untreated, heart failure leads to the pump twin's death in 50 to 75 percent of cases. The rate of survival of the pump twin is related to weight. If the weight of the acardiac twin is larger than the pump twin by 50 percent or more, death occurs in 65 percent of cases. If the weight of the acardiac twin is larger than the pump twin by 75 percent or more, death occurs in 95 percent of cases. 

An additional concern for TRAP sequence twins, as for all monochorionic twins, is the complications they face if they share the same amniotic sac. Twins who share both a placenta and an amniotic sac are called monochorionic-monamniotic twins. Monochorionic-monamniotic twins are at an increased risk of entangling and twisting the umbilical cord, which can result in sudden death.

Close observation by ultrasound will be required throughout the pregnancy to monitor the heart health of the pump twin and the size of the acardiac twin. If signs of heart failure appear in the pump twin early in pregnancy, our team at the Fetal Concerns Center will discuss treatment options to help increase the chance of the pump twin's survival.

Our physicians at the Fetal Concerns Center also recommend ultrasound monitoring of amniotic fluid levels throughout the pregnancy. As the pump twin works harder and harder to meet blood supply demands, blood flow to the kidneys increases. This leads to an excess of fetal urine, the primary source of amniotic fluid.

More than half of TRAP sequence pregnancies are affected by excessive amniotic fluid or polyhydramnios, which can lead to early labor and premature delivery. In general, about 75 percent of TRAP sequence pregnancies are complicated by premature labor.

There are two options for delivery of TRAP sequence pregnancy: 

• Normal delivery: If the acardiac twin is small and the pump twin's heart does not seem stressed, the pregnancy may lead to a normal delivery. Delivery may or may not require special measures or a cesarean procedure , depending on the healthy twin's needs. It is advised, however, that delivery be at a hospital with a Neonatal Intensive Care Unit (NICU) and trained experts who can quickly respond if specialized care is needed. 
• Early delivery: If the pregnancy is far enough along (28 weeks or more) and ultrasounds reveal that the pump twin is showing signs of heart failure, an early delivery may be recommended. In this case, it is best for delivery to take place at a hospital that specializes in high-risk births with a NICU.

While all TRAP sequence pregnancies require careful monitoring, not all require treatment.

When treatment is recommended, the goal is to preserve the life of the healthy twin. If the pregnancy has progressed beyond 28 weeks, delivery of the healthy twin may be recommended without additional treatments.

If the health risk for the pump twin becomes high enough early in pregnancy, other prenatal treatments may be recommended to stop the extra pumping between the twins. These procedures, performed using highly specialized techniques within the womb, are performed to block the blood flow to the acardiac twin to allow the healthy twin to survive and develop as normally as possible.

The preferred prenatal treatment method for sealing off the blood flow to the acardiac twin is radiofrequency ablation (RFA), a procedure performed by our team of experts at the Fetal Concerns Center. RFA is usually performed between 17 and 24 weeks of pregnancy.

Using spinal anesthesia for the mother and ultrasound guidance, physicians perform RFA with a thin needlelike device that is placed through the mother's abdomen and uterus into the abnormal twin's umbilical cord. Heat energy is then applied to stop the blood flow to the cord. RFA has been shown to be extremely successful, with a 90 percent or higher survival rate for the normal twin when delivered after 35 weeks into the pregnancy.

After RFA or any prenatal treatment, mothers are typically admitted to the hospital for monitoring for two to three days. They are then discharged for ongoing care by their obstetrician. Bed rest may be recommended and medications prescribed to prevent premature delivery. Ongoing ultrasound surveillance also is recommended.

Yes, most pump twins are born without significant problems. So care of your newborn should be the same as caring for any newborn. Physicians will thoroughly examine your baby immediately following delivery. If there are no complications, you and your loved ones should be able to see and hold your newborn right away. If your baby does have special needs, he or she will be taken to the NICU for treatment by neonatologists, who specialize in caring for critically ill newborns.

This depends on the condition of your baby. If your baby is healthy, he or she will go home after delivery within the standard timeframe of your stay. If there are complications that require more intensive treatment or prolonged assessment, your baby may need to stay in the hospital longer.

Long-term outcomes for pump twins are quite favorable, with no related health concerns in the future.

Future pregnancies will not be affected by a prior TRAP sequence pregnancy. A TRAP sequence pregnancy does not run in families and there is no documented reports of recurrence in women.