TAPS and TRAP sequence

Definition

• TAPS (twin anaemia–polycythaemia sequence) is a complication of monochorionic twins caused by minute (less than 1 mm) arteriovenous anastomoses through which blood flows slowly from donor to recipient; the result is highly discordant haemoglobin levels without amniotic fluid discordance.
• TRAP sequence (twin reversed arterial perfusion) is a complication of monochorionic multiple pregnancy in which an acardiac twin is perfused by a structurally normal pump twin through arterio-arterial anastomoses.

Classification

TAPS (Twin Anaemia-Polycythaemia Sequence)

• TAPS is caused by a few small arteriovenous anastomoses, which lead to a slow, chronic transfusion without amniotic fluid discordance, which is why it is often missed without a targeted search.
• The diagnosis is made using the maximum systolic velocity of the middle cerebral artery: in the anaemic donor, the value is above 1.5 MoM and in the polycythaemic recipient, it is below 1.0 MoM.

TRAP sequence (Acardiac Twin)

• In TRAP (Twin Reversed Arterial Perfusion) sequence, a malformed twin without a functioning heart is perfused in a retrograde manner by the pump twin via a large arterio-arterial anastomosis.
• The pump twin is at risk of developing heart failure, hydrops and polyhydramnios.

Occurrence & epidemiology

Epidemiology

• TRAP sequence affects about 1% of monochorionic pregnancies and about one in 35,000 of all pregnancies.

Clinical features

Clinical features and complications

• In TAPS the risk of neurodevelopmental impairment is increased, more so in the donor than in the recipient; after spontaneous TAPS, impairment was found in 26% of survivors.
• In TRAP the pump twin is at risk of high-output cardiac failure from the hyperdynamic circulation, shown for example by hydrops fetalis and polyhydramnios.

Diagnosis

• Prenatally, TAPS is diagnosed from discordant middle cerebral artery Doppler findings; by Delphi consensus, a peak systolic velocity of at least 1.5 MoM in the donor and at most 0.8 MoM in the recipient is used.
• Pathognomonic for TRAP is reversed blood flow into the acardiac twin through its umbilical artery on colour Doppler.
• The acardiac twin shows multiple severe malformations, subcutaneous oedema and no heart; the finding may already be evident in the first trimester.

Keep learning in the app

Further reading (selection)

1. Ultrasound Obstet Gynecol 2025: ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy (PMC, open access)
2. DocCheck Flexikon, TRAP-Sequenz

Cross-references