A 28-year-old primigravida presents at 13 weeks gestation with a twin pregnancy. Transvaginal ultrasound shows a single placenta with an intertwin membrane that meets the placental surface at a 90-degree angle with no triangular placental projection. The membrane is thin (2 layers). The structure marked **B** in the diagram represents this sonographic appearance. Which of the following is the most important clinical implication of this chorionicity-amnionicity pattern?

Question

Accepted Answer

B. Requirement for fetoscopic laser photocoagulation of placental vascular anastomoses if twin-to-twin transfusion syndrome develops. ## Why Option 1 is correct

The T-sign appearance with a thin 2-layer intertwin membrane is pathognomonic for **monochorionic-diamniotic (MCDA) twins**, marked as **B** in the diagram. MCDA twins share a single placenta with vascular anastomoses (arterio-arterial, veno-venous, and critically, unbalanced arterio-venous connections). The most significant complication is twin-to-twin transfusion syndrome (TTTS), occurring in 10–15% of MCDA pregnancies, characterized by unidirectional vascular shunting causing a donor twin (anemia, growth restriction, oligohydramnios/"stuck twin") and a recipient twin (polycythemia, cardiomegaly, polyhydramnios). Fetoscopic laser photocoagulation of placental anastomoses is the definitive treatment and significantly improves perinatal survival. This is the single most important clinical implication of identifying MCDA chorionicity in the first trimester—early detection allows for appropriate surveillance (ultrasound every 2 weeks from 16 weeks) and timely intervention if TTTS develops.

## Why each distractor is wrong

- **Option 2**: MCDA twins carry substantially elevated risks compared to dichorionic-diamniotic twins, including TTTS (10–15%), selective fetal growth restriction, twin anemia-polycythemia sequence, and increased single-fetal demise risk. This statement directly contradicts the prognostic importance of chorionicity.
- **Option 3**: Universal umbilical cord entanglement is a feature of **monochorionic-monoamniotic (MCMA) twins** (marked as C), not MCDA. MCMA requires delivery at 32–34 weeks due to cord entanglement risk; MCDA uncomplicated pregnancies are delivered at 36–37 weeks.
- **Option 4**: The T-sign with a thin membrane is diagnostic of monochorionic (single) placentation, not dichorionic. The presence of a single placenta with vascular anastomoses is the defining feature that creates the risk of shared circulatory complications.

**High-Yield:** The T-sign (90-degree angle, no placental wedge, thin 2-layer membrane) = MCDA = shared placental vascular anastomoses = TTTS risk → biweekly surveillance from 16 weeks and readiness for laser photocoagulation.

[cite: ISUOG Guidelines on Twin Pregnancy 2024; ACOG Practice Bulletin 234 — Multifetal Gestations]

Answer

A. Universal umbilical cord entanglement necessitating delivery by 32-34 weeks

Answer

C. Guaranteed dichorionic placentation with minimal risk of shared vascular anastomoses

Answer

D. No increased risk of fetal complications compared to dichorionic-diamniotic twins

Explanation

Why Option 1 is correct

Why each distractor is wrong

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