Dichorionic-Diamniotic Twin Pregnancy
This is most likely a dichorionic-diamniotic (DCDA) twin pregnancy based on the presence of two anterior placental masses, which definitively confirms dichorionic placentation with 97-99% sensitivity and 95-100% specificity. 1
Key Diagnostic Features Supporting Dichorionic Placentation
The clinical presentation provides multiple converging indicators of dichorionicity:
Two separate anterior placentas is the most definitive finding, as the presence of two distinct placental masses confirms dichorionic placentation with near-perfect diagnostic accuracy 2, 1
Membrane thickness of 2.1 mm exceeds the 2 mm threshold that indicates dichorionic placentation, as monochorionic membranes are characteristically thinner (typically <2 mm) 1
Same gender twins does not exclude dichorionicity—while monochorionic twins are always monozygotic and therefore same-gender, approximately one-third of monozygotic twins are actually dichorionic when the morula cleaves within 4 days of fertilization 2
Why This Matters Clinically
The distinction between dichorionic and monochorionic twins has profound implications for perinatal outcomes and surveillance intensity:
Dichorionic twins have approximately half the perinatal mortality of monochorionic twins (1.6% vs 7.6% fetal demise rate) 2, 1
Twin-twin transfusion syndrome (TTTS) risk is markedly lower in DCDA pregnancies, as TTTS affects 8-12% of monochorionic-diamniotic pregnancies but is essentially absent in dichorionic twins who lack shared placental vascular anastomoses 1
Standard growth assessments for fetal concordance are appropriate for DCDA twins, rather than the intensive every-2-week ultrasound surveillance required for monochorionic pregnancies starting at 16 weeks 3, 1
Common Pitfall to Avoid
Do not misclassify a bilobed monochorionic placenta as dichorionic simply because two placental masses appear present on ultrasound. At 19 weeks, the lambda sign (which indicates dichorionicity) becomes less reliable as it regresses with advancing gestational age 2. However, in this case, the combination of two truly separate anterior placentas AND membrane thickness >2 mm provides robust confirmation of dichorionicity 1.