From the Guidelines
Nuchal translucency (NT) difference is a more predictive early finding of twin to twin transfusion syndrome (TTTS) than crown-rump length (CRL) difference. When evaluating monochorionic twin pregnancies, it is crucial to consider the predictive value of various ultrasound parameters for TTTS. According to the study by 1, both nuchal translucency abnormalities and crown-rump length discrepancies have been associated with an increased risk of twin-twin transfusion syndrome. However, an NT discordance >20% has been associated with a >30% risk of severe twin-twin transfusion syndrome and early fetal death, making it a more significant predictor than CRL difference. Some key points to consider in the evaluation of monochorionic twin pregnancies include:
- Increased nuchal translucency measurement at 11 to 14 weeks of gestation for single or twin gestations can be an early manifestation of interfetal transfusion and early hypervolemic congestion in a recipient twin 1.
- A crown-rump discrepancy of >10% is associated with almost a doubling the risk of developing twin-twin transfusion syndrome, but it is less specific than NT discordance 1.
- Serial US surveillance of monochorionic twins is advised because of complications associated with sharing a single placenta, such as twin-twin transfusion syndrome, unequal placental sharing with discordant twin growth and selective intrauterine growth restriction, and twin anemia polycythemia sequence 1. Given the potential for significant morbidity and mortality associated with TTTS, early detection and monitoring are crucial, and first-trimester screening that identifies significant NT discrepancy should prompt increased surveillance with more frequent ultrasounds to monitor for TTTS development 1.
From the Research
Predictive Findings for Twin to Twin Transfusion Syndrome
The early findings of Crown Rump Length (CRL) difference and Nuchal Translucency (NT) difference are assessed as predictors of Twin to Twin Transfusion Syndrome (TTTS) in monochorionic diamniotic twin pregnancies.
- Studies have investigated the correlation between these discrepancies and the subsequent development of TTTS.
- A study published in 2009 2 found that a NT discordance of 20% or more best predicted TTTS development, with a sensitivity of 64% and a specificity of 78%.
- In contrast, a 2012 study 3 found that CRL discrepancy performed well as a screening test for selective fetal growth restriction (sFGR), but not for TTTS.
- Another study in 2016 4 determined that NT discordance was more discriminatory of adverse outcomes, including TTTS, than CRL discordance.
- A 2011 study 5 found that discordance in CRL and NT measured during the first trimester scan was not a clinically useful predictor of the subsequent development of TTTS.
- A more recent study in 2020 6 found that although discordant first-trimester CRL and NT in monochorionic twins are poor screening tools for early prediction, if positive, they increase the risk of developing complications, including TTTS.
Comparison of CRL and NT Discordance
- The studies suggest that NT discordance may be a more predictive finding of TTTS than CRL discordance, with a study in 2009 2 finding a positive predictive value of 50% and negative predictive value of 86% for TTTS development if NT discordance was 20% or more.
- However, the predictive value of NT discordance is not consistent across all studies, with some finding it to be a poor screening tool for early prediction of TTTS 5, 6.
- CRL discordance, on the other hand, has been found to be a better predictor of sFGR than TTTS 3.
Summary of Findings
- NT discordance of 20% or more may be a predictive finding of TTTS, with a sensitivity of 64% and a specificity of 78% 2.
- NT discordance is more discriminatory of adverse outcomes, including TTTS, than CRL discordance 4.
- CRL discordance is a better predictor of sFGR than TTTS 3.
- Discordance in CRL and NT measured during the first trimester scan is not a clinically useful predictor of the subsequent development of TTTS 5.