Twin Reversed Arterial Perfusion sequence (TRAPs) is a very rare complication that is specific to monochorionic pregnancies (babies who share a placenta) and occurs in roughly 1 percent of these pregnancies.
This occurs after one fetus develops normally whilst the other fails to develop but there are blood vessels that form in the placenta, connecting them. This results in the normal baby pumping blood directly into the fetus that didn’t develop, which becomes an abnormal appearing mass that continues to grow (undeveloped fetus). The normal baby will eventually become tired and begin to show signs of strain.
If your Obstetrician suspects that you may have this condition, you will be referred to a specialist fetal medicine centre for further evaluation and treatment, which usually involves laser therapy, to stop the blood flowing into the undeveloped fetus.
Following the procedure, the normal baby will continue to be monitored closely. Occasionally, this is discovered later in the pregnancy, and provided the normal baby is well, you may be closely monitored without intervention.
The diagrams below illustrate TRAPS in a twin and triplet pregnancy.