# Twin Reversed Arterial Perfusion and Selective Fetal Growth Restriction

> NEET-PG Obstetrics and Gynaecology notes on twin reversed arterial perfusion, covering pump twin physiology, cord ablation and selective growth restriction.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/twin-reversed-arterial-perfusion-and-selective-fgr
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Williams Obstetrics; Dutta's Textbook of Obstetrics
- How to cite: "Twin Reversed Arterial Perfusion and Selective Fetal Growth Restriction", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/twin-reversed-arterial-perfusion-and-selective-fgr

## Direct answer

Twin reversed arterial perfusion, or TRAP sequence, is a rare monochorionic twin complication in which an acardiac twin without a functioning heart is perfused by blood pumped from a normal co-twin, the pump twin. The pump twin risks high-output heart failure, polyhydramnios and preterm birth. In selected cases the acardiac twin's cord is ablated by radiofrequency, bipolar coagulation or laser, usually in the second trimester, to protect the pump twin.

## What you must remember

- **Mechanism:** arterio-arterial anastomoses in a shared placenta allow reverse flow from the pump twin into the acardiac twin.
- **Acardiac twin:** lacks a functioning heart and is often headless, classically described as acardius acephalus.
- **Pump twin:** develops high-output heart failure with cardiomegaly, hydrops and polyhydramnios, and carries a high risk of death if the sequence is untreated.
- **Diagnosis:** monochorionicity on early scan, an acardiac twin with absent cardiac activity, and reversed flow in the acardiac cord.
- **Treatment:** ablation of the acardiac twin's umbilical cord, usually in the second trimester, to stop the reverse perfusion.
- **Selective FGR:** a monochorionic pregnancy in which one twin has a small estimated weight (below the 10th centile), usually with discordance of about 25 per cent or more.
- **Gratacós types:** type I has normal umbilical artery Doppler, type II has persistent absent or reversed end-diastolic flow, and type III has intermittent absent or reversed flow, with type II at highest risk of deterioration and type III unpredictable, including sudden death of the smaller twin.

## Sorting a suspected TRAP pregnancy

A monochorionic pregnancy with an abnormal fetus needs a systematic assessment, because the survival of the pump twin is the main target. The steps below follow the usual sequence.

1. **Confirm monochorionicity:** check the placental type early, since a T sign indicates a monochorionic placenta and a lambda sign a dichorionic one.
2. **Identify the acardiac twin:** look for absent cardiac activity, a grossly abnormal body and reversed flow in the acardiac cord.
3. **Assess the pump twin:** check for cardiomegaly, tricuspid regurgitation, hydrops and polyhydramnios, which signal heart failure.
4. **Judge the risk:** a large acardiac twin relative to the pump twin raises the risk to the pump twin, and this guides the urgency of treatment.
5. **Refer for ablation:** send the pregnancy to a fetal medicine centre, where ablation of the acardiac twin's umbilical cord is usually performed in the second trimester.
6. **Follow the pump twin:** monitor growth, Doppler and fluid volume after treatment, and prepare for possible preterm birth.

## Where the exam tries to catch you

Questions here test whether you target the pump twin and separate TRAP from selective growth restriction.

- **Trap:** Assuming the acardiac twin needs no treatment because it cannot survive. — **Correct:** The acardiac twin drains the pump twin's circulation, so treatment aims to save the pump twin.
- **Trap:** Treating every discordant monochorionic pair with laser surgery. — **Correct:** Selective FGR is managed mainly with surveillance and timed delivery, and fetal surgery is reserved for selected cases.
- **Trap:** Ignoring the placental vessel connections in a monochorionic pair. — **Correct:** Arteriovenous anastomoses explain twin-twin transfusion syndrome, while arterio-arterial anastomoses explain TRAP, so placental anatomy is central to diagnosis.

## Frequently asked questions

### What causes twin reversed arterial perfusion sequence?
Twin reversed arterial perfusion sequence arises from arterio-arterial anastomoses in the shared placenta of a monochorionic twin pregnancy. These connections allow blood from the pump twin to flow backwards into the acardiac twin.

### Why does the pump twin suffer in TRAP sequence?
The pump twin's heart must pump blood to itself and to the acardiac twin, which causes high-output heart failure. Cardiomegaly, hydrops, polyhydramnios and preterm birth may follow if the sequence is untreated.

### How is the acardiac twin treated in TRAP sequence?
The acardiac twin is treated by ablating its umbilical cord with radiofrequency, bipolar coagulation or laser, usually in the second trimester. The aim is to stop the reverse flow and protect the pump twin.

### How is selective fetal growth restriction defined in monochorionic twins?
Selective fetal growth restriction is defined by a small twin with estimated fetal weight below the 10th centile and discordance of about 25 per cent or more in a monochorionic pregnancy. It is classified as type I, II or III using umbilical artery Doppler findings.