Can we still label the twins as dichorionic‑diamniotic (DCDA) based only on the appearance of two separate placental lobes?

Chorionicity Determination with Two Separate Placental Lobes

No, you cannot definitively label twins as dichorionic-diamniotic (DCDA) based solely on the appearance of two separate placental masses, as approximately 3% of monochorionic placentas present as bipartite (two separate lobes) and can still harbor vascular anastomoses that cause twin-to-twin transfusion syndrome. 1

Critical Diagnostic Approach

The presence of two separate placental masses is suggestive but not diagnostic of dichorionicity, particularly when evaluated in the second trimester. 2

Why Two Placentas Can Be Misleading

• Bipartite monochorionic placentas occur in nearly 3% of monochorionic twin pregnancies, appearing as two completely separate placental masses connected only by membranes. 1

• These bipartite monochorionic placentas can still contain vascular anastomoses between the twins, with documented cases developing twin-to-twin transfusion syndrome despite the separated appearance. 1

• By the second trimester, adjacent dichorionic placentas may appear fused into a single mass, while truly monochorionic placentas may appear separated—making placental number an unreliable sole criterion. 2

Definitive Diagnostic Criteria

First Trimester Assessment (Most Reliable)

The assessment of chorionicity by first trimester ultrasound when other reliable signs can be observed is crucial. 2

• Count the number of gestational sacs at 6-10 weeks, which equals the number of chorions with nearly 100% accuracy. 2, 3

• Evaluate the intertwin membrane-placental junction at 11-14 weeks:

  • Lambda or "twin peak" sign = dichorionic (chorionic tissue extends between the amnions) 2, 3, 1
  • T-sign = monochorionic (amnion inserts flush against placenta without intervening chorion) 2, 3, 1

• The T-sign is the most specific marker for monochorionicity and prevents misclassification of bilobed placentas as dichorionic. 4

Second Trimester Evaluation (Less Reliable)

When chorionicity was not established in the first trimester:

• Membrane thinning, loss of the lambda sign, and apparent placental fusion make reliable determination difficult in the second trimester. 2

• Detection of discordant external genitalia (one male, one female) definitively indicates dizygotic and therefore dichorionic gestation, but this only applies to ~45% of twins. 2

• Two separate placental masses "help establish" but do not confirm dichorionicity. 2

Clinical Implications of Misclassification

If Monochorionic Twins Are Mislabeled as Dichorionic:

• Monochorionic twins have approximately 2-fold higher perinatal mortality (7.6% vs 1.6% fetal demise) compared to dichorionic twins due to shared vascular anastomoses. 4

• Monochorionic twins require intensive ultrasound surveillance every 2 weeks starting at 16 weeks to detect twin-to-twin transfusion syndrome, selective fetal growth restriction, and twin anemia-polycythemia sequence. 2, 4

• Dichorionic twins are typically followed every 3-4 weeks without concern for placental sharing complications. 2

If Dichorionic Twins Are Mislabeled as Monochorionic:

• Unnecessary intensive surveillance and parental anxiety
• Inappropriate clinical management decisions

Recommended Diagnostic Algorithm

1. If first trimester ultrasound was performed: Review the original images for number of gestational sacs and membrane insertion (lambda vs T-sign). 2, 3

2. If no first trimester documentation exists and you see two placental masses in second trimester:

   • Carefully evaluate the intertwin membrane insertion site for residual lambda or T-sign 2
   • Assess fetal gender—if discordant, confirms dichorionicity 2
   • If same gender or gender not yet determined, consider the pregnancy potentially monochorionic until proven otherwise 1

3. When uncertainty persists:

   • Implement monochorionic surveillance protocols (every 2 weeks starting at 16 weeks) as the safer approach 4
   • Consider referral to a maternal-fetal medicine specialist for expert evaluation 2

Common Pitfalls to Avoid

• Never rely on placental number alone to determine chorionicity, especially in the second trimester. 2, 1

• Do not assume that two separate placental masses exclude monochorionicity—bipartite monochorionic placentas exist and carry the same risks as single-mass monochorionic placentas. 1

• Avoid using a single sonographic feature; always use a combination of findings (gestational sac number, membrane characteristics, placental number, fetal gender). 2, 3