A 32-year-old G2P1 woman presents for a routine first-trimester ultrasound at 11+3 weeks of gestation. The scan reveals two gestational sacs, each containing a live fetus with appropriate crown-rump length. The structure marked **C** in the diagram is noted to be thick (2 mm) and inserts into the placenta with a triangular projection of placental tissue visible at the insertion point. Which of the following best describes the significance of this finding for antenatal management?

Question

A 32-year-old G2P1 woman presents for a routine first-trimester ultrasound at 11+3 weeks of gestation. The scan reveals two gestational sacs, each containing a live fetus with appropriate crown-rump length. The structure marked **C** in the diagram is noted to be thick (>2 mm) and inserts into the placenta with a triangular projection of placental tissue visible at the insertion point. Which of the following best describes the significance of this finding for antenatal management?

Accepted Answer

A. Confirms dichorionic diamniotic gestation, necessitating ultrasound surveillance every 4 weeks from 24 weeks and delivery planning at 37–38 weeks. ## Why Option 1 is correct

The thick intertwin membrane (>2 mm) inserting into the placenta with a triangular placental tissue projection—the **lambda sign** or "twin peak"—is pathognomonic of **dichorionic diamniotic (DCDA) twin gestation**. This finding, best identified in the first trimester (11–14 weeks), is the single most important determinant of chorionicity and directly dictates risk stratification and surveillance strategy. DCDA twins result from fertilization of two ova or monozygotic splitting within the first 3 days; each fetus has its own placenta and amniotic sac, eliminating the unique complications of shared placental vasculature (TTTS, TAPS, selective IUGR). Per ACOG/SMFM guidelines, DCDA twins require serial ultrasound every 4 weeks from 24 weeks (anatomy at 20 weeks, growth at 24/28/32/36 weeks) and delivery at 37+0 to 38+0 weeks in uncomplicated cases. This contrasts sharply with monochorionic twins, which require more intensive surveillance and earlier delivery.

## Why each distractor is wrong

- **Option 2**: Monochorionic diamniotic (MCDA) twins are characterized by a **thin membrane** (the "T-sign") meeting the placenta at a right angle *without* placental tissue projection. The thick lambda sign definitively excludes MCDA; TTTS is a complication of MCDA, not DCDA.
- **Option 3**: Monochorionic monoamniotic (MCMA) twins result from splitting after day 8 and have a single amniotic sac (cord entanglement risk). The presence of two separate gestational sacs and the lambda sign exclude MCMA.
- **Option 4**: This option contradicts the imaging finding. The structure marked **C** is explicitly thick (>2 mm), not thin. A thin membrane would suggest MCDA, not DCDA, and would require biweekly (not less intensive) surveillance.

**High-Yield:** Lambda sign (thick intertwin membrane with triangular placental projection) = dichorionic diamniotic = two placentas, two amniotic sacs, lower perinatal risk, surveillance every 4 weeks from 24 weeks, delivery at 37–38 weeks.

[cite: ACOG/SMFM Multifetal Gestations 169; ISUOG Guidelines]

Answer

B. Demonstrates a thin intertwin membrane consistent with monochorionic diamniotic twins, allowing less intensive surveillance

Answer

C. Indicates monochorionic diamniotic gestation with high risk of twin-twin transfusion syndrome, requiring biweekly ultrasound from 16 weeks

Answer

D. Suggests monochorionic monoamniotic gestation with risk of cord entanglement, mandating delivery by 32–34 weeks

Explanation

Why Option 1 is correct

Why each distractor is wrong

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