Twin Reversed Arterial Perfusion (TRAP)
Diagram of twin reversed
What is twin reversed arterial perfusion (TRAP)?
This phenomenon occurs only in a twin pregnancy where the babies share the same placenta. The incidence is about 1 in every 35,000 identical twin births. One twin is normally developed and referred to as the 'pump' twin. He pumps blood for himself as well as for the other twin. The other twin is not fully developed and often lacks any personable features. This twin does not have a heart and is referred to as the 'acardiac' or abnormal twin. The term 'reversed perfusion' is used because the blood enters the undeveloped twin through the vessels in the opposite direction.
This sequence of events places the normal twin at risk for heart failure. Left untreated, there is a 50%-75% chance of mortality for the normal twin. The cause of TRAP is unknown.
What testing is recommended during the pregnancy?
Ultrasound examination can confirm the diagnosis and will be used to monitor the pregnancy. The size of both babies is an important determinant for the outcome of the normal twin. If the estimated weight of the abnormal twin is greater than 75% of the normal twin, the survival for the normal twin is about 10%. A fetal echocardiogram is necessary to evaluate the heart function of the developed twin.
An amniocentesis is offered to analyze the chromosomes of each twin. About 9% of the normal twins, and approximately 30% of the abnormal twins, will have a chromosomal abnormality.
Are there special considerations planned for the delivery?
It is recommended that the delivery is planned at a hospital that is prepared for high-risk deliveries, including a neonatal intensive care unit and a pediatric cardiologist. A Cesarean delivery is recommended when indicated for obstetrical reasons.
Is there fetal treatment for TRAP?
Treatment is available for select patients when the normal twin is showing signs of heart failure, or the acardiac twin is increasing in size as compared to the normal twin. A fetoscope, guided by ultrasound, is inserted into the abdomen to identify both umbilical cords. The umbilical cord supporting the undeveloped twin is occluded to stop the flow of blood.
Are there other treatments for TRAP?
In less severe cases, serial ultrasounds can monitor the normal twin for signs of distress. If a problem develops later in the pregnancy, early delivery may be an option.
Are there other health problems that are more common among babies with TRAP?
Babies with TRAP are at risk for preterm delivery and their health is usually dependent on gestational age. Some babies may have ongoing cardiac problems.
Can this happen again with another pregnancy?
TRAP is a rare condition with no familial link identified. An amniocentesis will provide karyotype information on both twins that could be a factor in future pregnancies. Your doctor and a genetic counselor will review the risk in your family.