What are the key ultrasound findings in the first trimester for monochorionic (mono-chorionic) diamniotic versus dichorionic (di-chorionic) diamniotic pregnancies?

From the Guidelines

Monochorionic diamniotic (MCDA) twin pregnancies can be distinguished from dichorionic diamniotic (DCDA) twin pregnancies through ultrasound findings, including the presence of a single placenta and a "T" sign in MCDA twins, whereas DCDA twins have separate placentas and a lambda or twin peak sign. The key ultrasound findings in the first trimester for MCDA versus DCDA pregnancies include the number of gestational sacs, the presence of separate and distinct amnions, and the intertwin membrane thickness 1. In MCDA pregnancies, the intertwin membrane is typically identified by 10 weeks on transvaginal ultrasound, and the absence of identification of the intertwin membrane can be technical, thus requiring confirmation either by demonstrating umbilical cord entanglement or by short-term serial ultrasound 1.

In contrast, DCDA pregnancies have separate placentas and amniotic sacs, with a lambda or twin peak sign visible on ultrasound, indicating the presence of two separate placentas 1. The determination of chorionicity is most accurate in the first trimester, with a reported accuracy of nearly 100% when the number of gestational sacs equals the number of chorions 1.

The importance of early determination of chorionicity cannot be overstated, as it dictates the frequency of monitoring, timing of delivery, and anticipation of potential complications, including twin-to-twin transfusion syndrome (TTTS), selective intrauterine growth restriction (sIUGR), and twin anemia-polycythemia sequence (TAPS) 1. Early determination of chorionicity, ideally before 14 weeks, is crucial for proper management of twin pregnancies, and ultrasound surveillance for TTTS should begin at 16 weeks of gestation for all MCDA twin pregnancies and continue at least every 2 weeks until delivery 1.

The ultrasound findings in the first trimester can help identify potential complications early on, allowing for closer monitoring and timely intervention, which can significantly improve outcomes for MCDA and DCDA twin pregnancies 1.

Some key points to consider when evaluating twin pregnancies include:

• The number of gestational sacs and placentas
• The presence of separate and distinct amnions
• The intertwin membrane thickness and presence of a "T" sign or lambda sign
• The presence of umbilical cord entanglement or short-term serial ultrasound to confirm the absence of an intertwin membrane
• The determination of chorionicity, which is most accurate in the first trimester
• The importance of early determination of chorionicity for proper management and anticipation of potential complications.

Overall, the key ultrasound findings in the first trimester for MCDA versus DCDA pregnancies can help identify potential complications early on, allowing for closer monitoring and timely intervention, which can significantly improve outcomes for twin pregnancies 1.

From the Research

Key Ultrasound Findings in Monochorionic Diamniotic Pregnancies

• The key ultrasound findings in monochorionic diamniotic pregnancies include the presence of a single placenta and two separate amniotic sacs 2.
• Ultrasound is essential for detecting and managing complications such as twin-to-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and selective intrauterine growth restriction (sIUGR) 2, 3.
• The diagnosis of TTTS is typically made before 26 weeks of gestation and is characterized by a significant amniotic fluid difference between the two fetuses 3.
• TAPS is a severe hemoglobin difference that occurs mainly after 26 weeks in previously uncomplicated pregnancies 3.
• Selective intrauterine growth restriction (sIUGR) is characterized by an intertwin growth difference of more than 20% and is often due to uneven placental sharing or placental dysfunction 3.

Comparison with Dichorionic Diamniotic Pregnancies

• Dichorionic diamniotic pregnancies have two separate placentas and two separate amniotic sacs, which reduces the risk of complications such as TTTS and TAPS 2, 4.
• Monochorionic diamniotic pregnancies have a higher risk of complications and a lower survival rate compared to dichorionic diamniotic pregnancies 4.
• Ultrasound findings in dichorionic diamniotic pregnancies are generally less complex and less likely to be associated with complications such as TTTS and TAPS 2.

Ultrasound Findings in the First Trimester

• The first trimester is critical for determining chorionicity and identifying potential complications in monochorionic diamniotic pregnancies 4.
• Ultrasound findings in the first trimester can help identify a subgroup of monochorionic twins with a high risk of complications and a low survival rate 4.
• Early detection of complications in the first trimester can help guide management and improve outcomes for monochorionic diamniotic pregnancies 4.