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

From the Guidelines

Accurate determination of chorionicity in the first trimester is crucial, and the key ultrasound findings for monochorionic diamniotic (MCDA) versus dichorionic diamniotic (DCDA) pregnancies are the "T-sign" for MCDA and the "lambda" sign for DCDA, with a reported sensitivity of 97% to 99% and specificity of 95% to 100% for the lambda sign. The presence of either a lambda sign or 2 separate placentas indicates DC placentation, while the "T" sign describes the ultrasound visualization of the perpendicular attachment of the intervening twin membrane to the placenta in MCDA gestations 1.

In the first trimester, the main goal of imaging is to confirm the presence and location of the gestational sacs, determine chorionicity and amnionicity, provide pregnancy dating, and document embryonic or fetal cardiac activity 1. The intertwin membrane is typically identified by 10 weeks on transvaginal US, and the absence of identification of the intertwin membrane can be technical, thus it is essential to confirm the absence either by demonstrating umbilical cord entanglement or by short-term serial US 1.

Some key points to consider when evaluating chorionicity include:

• The number of placentas
• The presence of a "lambda" or "twin peak" sign, which is seen in DC gestations
• The presence of a "T" sign, which is seen in MC gestations
• The thickness of the dividing membrane
• The use of a combination of features to accurately determine chorionicity, rather than relying on a single feature 1.

It is also important to note that the determination of chorionicity is most accurate in the first trimester, and that the accuracy of this determination can be affected by the quality of the ultrasound imaging and the experience of the operator 1. We recommend routine first-trimester sonographic determination of chorionicity and amnionicity, as this is crucial for appropriate risk assessment, counseling, and management planning throughout the pregnancy 1.

From the Research

Key Ultrasound Findings in Monochorionic Diamniotic Pregnancies

• The presence of a lambda sign at the base of the intertwin membrane is indicative of a monochorionic diamniotic (MCDA) twin pregnancy 2
• Chorionic membrane folding (CMF) can occur in MCDA pregnancies, which can show two gestational sacs and a lambda sign, and the CMF can flatten or change during the pregnancy 2
• Ultrasound determination of chorionicity in the first trimester has a high accuracy, but it is associated with some pitfalls 2
• Intertwin differences in crown-rump length (CRL), nuchal translucency (NT) thickness, ductus venosus pulsatility index for veins (DV PIV), presence or absence of tricuspid regurgitation and right ventricular E/A ratio can be assessed to predict complications in MCDA twins 3
• NT discordance of ≥20% is the best predictive marker for twin-twin transfusion syndrome (TTTS) in MCDA twins, with a receiver operating characteristic (ROC) area under the curve (AUC) of 0.79 3

Key Ultrasound Findings in Dichorionic Diamniotic Pregnancies

• No specific ultrasound findings are mentioned in the provided studies for dichorionic diamniotic pregnancies

Comparison of Ultrasound Findings in Monochorionic and Dichorionic Diamniotic Pregnancies

• Monochorionic diamniotic pregnancies are at increased risk of adverse outcomes due to unequal placental sharing and placental vascular communications between the fetal circulations 3, 4, 5
• Ultrasound examination in the first trimester and at 16 weeks' gestation can predict fetal complications in monochorionic diamniotic twin pregnancies, including twin-twin transfusion syndrome and selective intrauterine growth restriction 4
• The placental characteristics of monochorionic diamniotic twin pairs with early- and late-onset discordant growth can be assessed by injection of each cord vessel with dyed barium sulphate and X-ray angiogram 5
• Ultrasound is essential for the detection and management of conditions that can complicate both dichorionic and monochorionic twin pregnancies, including monoamnionicity, conjoined twins, twin reversed arterial perfusion (TRAP) syndrome, and twin anemia-polycythemia sequence (TAPS) 6