From the Guidelines
The key ultrasound (US) differences between monochorionic diamniotic (MCDA) and dichorionic diamniotic (DCDA) gestations involve several distinct features, including a single placenta with a thin dividing membrane (T-sign) in MCDA twins and two separate placentas or a single fused placenta with a thick dividing membrane (lambda or twin peak sign) in DCDA twins, as noted in the most recent study by 1.
Key Differences
- In MCDA twins, the membrane appears as two thin layers without tissue between them, whereas DCDA twins show a thicker membrane with tissue between the layers.
- Gender concordance is always present in MCDA twins (always same sex), while DCDA twins may be same or different sex.
- MCDA twins have a higher risk for complications, including twin-to-twin transfusion syndrome, selective intrauterine growth restriction, and twin anemia-polycythemia sequence, requiring more frequent monitoring, as highlighted in 1 and 1.
Determination of Chorionicity
- These differences are most reliably determined in the first trimester, ideally before 14 weeks gestation, as the lambda sign may disappear later in pregnancy, as noted in 1 and 1.
- Accurate determination of chorionicity is crucial for pregnancy management, as MCDA pregnancies require more intensive surveillance due to their higher complication rates, as emphasized in 1 and 1.
Monitoring and Surveillance
- MCDA twins require more frequent monitoring, with ultrasound surveillance starting at 16 weeks of gestation and continuing at least every 2 weeks until delivery, as recommended in 1.
- The monitoring should include assessment of amniotic fluid volumes, evaluation of fetal growth, and Doppler study of the umbilical arteries, as suggested in 1 and 1.
From the Research
Ultrasound Differences between MCDA and DCDA Gestations
The key ultrasound differences between monochorionic-diamniotic (MCDA) and dichorionic-diamniotic (DCDA) gestations are:
- Placental morphology: MCDA pregnancies have a single placenta with intertwin vascular anastomoses, whereas DCDA pregnancies have two separate placentas 2, 3, 4.
- Umbilical cord insertion and abnormalities: Abnormal cord insertion and hypo-/hypercoiling are more common in MCDA pregnancies 4.
- Fetal vascular malperfusion lesions: MCDA pregnancies have a higher incidence of fetal vascular malperfusion lesions, which can lead to adverse neonatal outcomes 4.
- Intertwin membrane thickness: The intertwin membrane is typically thinner in MCDA pregnancies due to the shared placenta 3, 5.
- Growth restriction and discordance: MCDA pregnancies are at higher risk for growth restriction and discordance due to the unequal sharing of the placenta and intertwin vascular anastomoses 3, 5, 6.
Complications and Outcomes
MCDA pregnancies are associated with higher rates of complications, including: