At 19 weeks gestation, with two anterior placental masses and an intertwin membrane measuring 2.1 mm, are the twins dichorionic‑diamniotic or monochorionic‑diamniotic?

Chorionicity Determination at 19 Weeks with Two Anterior Placentas and 2.1mm Membrane

These twins are most likely DCDA (dichorionic-diamniotic), but the diagnosis cannot be definitively confirmed by placental appearance alone at 19 weeks, and first-trimester imaging should be reviewed to avoid potentially dangerous misclassification.

Critical Diagnostic Limitations at 19 Weeks

The presence of two separate anterior placental masses is suggestive but not definitive for dichorionicity at this gestational age 1. Here's why this matters clinically:

• Monochorionic placentas can appear as two separate masses in approximately 3% of cases (bipartite monochorionic placentas), and these still carry all the serious risks of typical monochorionic twins including twin-twin transfusion syndrome 2, 3
• By the second trimester, adjacent dichorionic placentas may appear fused while truly monochorionic placentas can appear separated, making placental number an unreliable sole criterion 1
• The lambda sign may be lost and membranes thin by 19 weeks, reducing diagnostic accuracy 4

Membrane Thickness Analysis

The 2.1mm intertwin membrane measurement provides limited diagnostic value:

• Membrane thickness >2mm suggests dichorionicity 5, and your measurement of 2.1mm falls just above this threshold
• However, membrane thickness measurements have high intraobserver and interobserver variability in the second and third trimesters, with 95% of repeated measurements varying by up to 114% 6
• Membrane thickness alone should never be used as the sole criterion for chorionicity determination 4, 1

Required Diagnostic Algorithm

To definitively determine chorionicity, you must:

1. Review first-trimester ultrasound images immediately (if available):

   • Count the number of gestational sacs at 6-10 weeks (≈100% sensitivity for chorionicity) 1
   • Look for lambda sign (twin-peak sign) at 11-14 weeks indicating dichorionicity 4
   • Look for T-sign at membrane-placental junction indicating monochorionicity 4, 2

2. At the current 19-week scan, evaluate:

   • Fetal external genitalia: Discordant gender (one male, one female) definitively confirms dizygotic and therefore DCDA twins 4, 1
   • Membrane insertion site: Look for any residual lambda or T-sign at the placental-membrane junction 1
   • Placental characteristics: Examine whether the two masses are truly separate or potentially bilobed 2, 3

3. If uncertainty persists after steps 1 and 2:

   • Manage as potentially monochorionic until proven otherwise 1
   • Implement intensive surveillance protocol with ultrasounds every 2 weeks starting immediately to screen for twin-twin transfusion syndrome 4
   • Refer to maternal-fetal medicine specialist for expert evaluation 1

Clinical Implications of Misclassification

The stakes are extremely high:

• Monochorionic twins have 2-fold higher perinatal mortality (7.6% vs 1.6% fetal demise rate) compared to dichorionic twins 4, 5
• 8-12% of MCDA pregnancies develop twin-twin transfusion syndrome, which is essentially absent in DCDA twins 4, 5
• Mislabeling MCDA twins as DCDA leads to inadequate surveillance and missed life-threatening complications 1
• Conversely, mislabeling DCDA as MCDA causes unnecessary intensive surveillance and parental anxiety 1

Surveillance Based on Final Determination

If confirmed DCDA:

• Standard growth assessments every 3-4 weeks 5
• No need for intensive twin-twin transfusion syndrome surveillance 5

If MCDA or uncertain:

• Ultrasound every 2 weeks beginning immediately 4
• Monitor for oligohydramnios-polyhydramnios sequence (maximal vertical pocket <2cm in one sac and >8cm in the other) 4
• Assess for thin dividing membrane and common anterior placental mass as TTTS risk factors 4

Common Pitfalls to Avoid

• Never rely solely on placental number for chorionicity determination, especially after the first trimester 1, 2
• Do not assume two separate placentas exclude monochorionicity—bipartite monochorionic placentas exist and carry identical risks 2, 3
• Avoid using membrane thickness as a standalone criterion given its poor reproducibility 6
• Always use a combination of features rather than any single sonographic finding 4, 1