Monochorionic twin pregnancy complications: TRAP sequence

Pathology description

The twin reversed arterial perfusion (TRAP) sequence is a condition that occurs in twins that share a single placenta (monochorionic), in which there is a twin with severe malformations (the TRAP twin) that make it incompatible with life. The TRAP twin is usually missing the heart (it is acardiac) and the head (it is acephalous).

The TRAP twin's umbilical cord does not receive blood from the placenta, but directly from its twin through a single large-calibre arterio-arterial communication established with the healthy twin's umbilical cord. In this way, the healthy twin is the one who sends the blood (which is why it is also called a ‘pump’ twin) to the acardiac twin and keeps it 'alive', despite its having no heart. This imposes a huge strain on the healthy twin's heart, which can lead to heart failure. If untreated, the healthy twin will die in more than 50% of cases. This risk increases proportionally with the size of the TRAP twin.

The TRAP twin receives low oxygenated blood that arrives from the healthy twin through the umbilical artery and supplies asymmetrically and preferentially the lower half of the body. This asymmetric arrival of blood to the TRAP twin results in severe developmental anomalies, often causing an absence of the organs of the upper hemibody (heart, trunk, head and upper extremities). In most cases, the TRAP twin is an amorphous, hydropic mass, with cystic areas, subcutaneous oedema and no heart.

The TRAP sequence affects approximately 1% of monochorionic pregnancies, with an incidence of 1 in 25,000 pregnancies.

What consequences can TRAP sequence have for the healthy fetus? And for the mother?

By definition, the TRAP twin has malformations incompatible with life. It can be large and, because its circulation depends directly on the healthy fetus, it will produce a cardiac overload to the healthy twin that can lead to its death in up to 50% of cases.

Diagnostic

Before the diagnosis of a TRAP sequence, we must assess its size and perform the following procedures on the healthy fetus: 

• Exhaustive ultrasound to rule out malformations, since these are more frequent in monochorionic pregnancies, that is, in those with a shared placenta. 
• Functional echocardiography with specific study of blood flow in the heart and fetal vessels to diagnose early signs of cardiac overload and risk of developing hydrops fetalis (accumulation of fluid in various areas of the fetus), which occurs prior to fetal death. 
• Amniocentesis to rule out genetic abnormalities. 

Due to the cardiac overload of the healthy fetus, there is an increase in the volume of urine and therefore an increase in the amount of amniotic fluid (polyhydramnios), which can lead to a very premature birth or miscarriage (if it is before the time when the fetus can survive). Therefore, in the follow-up we must always control the cervical length of the pregnant woman by ultrasound.

Treatment

The goal of fetal surgery is to disrupt the blood supply from the healthy twin to the TRAP twin, which could never survive due to its severe malformations. This reduces the healthy twin's heart overload and increases its chances of survival.

Therefore, the TRAP sequence requires the closure of the TRAP twin’s cord. This can be performed by laser coagulation or with bipolar clamp by combined control (ultrasound and fetoscopy). 

What appointments are necessary for fetal surgery?

The hospital admission of the pregnant woman will initially last 1-2 days, and then will require weekly checkups and ultrasound, and she must rest at home with little physical activity until the end of pregnancy, especially for the first 3-4 weeks after the intervention. Normally, delivery can be performed between 34 and 36 weeks. 

During pregnancy, you will receive the support of nurses specialized in fetal medicine, not only on a technical level but also on an emotional level throughout the process. In addition, we can put you in contact with other families who have gone through the same experience. This is very positive and helps to humanize and understand the problem in a much more intuitive way and without the difficulties that sometimes arise when receiving only technical information from professionals.