First-Trimester Crown-Rump Length (CRL) and Nuchal Translucency (NT) Discordance in Monochorionic Twins: An Ominous Sign or a Benign Feature?

Background: This study aimed to evaluate the usefulness of the first-trimester crown-rump length (CRL) and nuchal translucency (NT) discordance in monochorionic diamniotic twins (MCDA) for the prediction of complications-twin-twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR) or intrauterine fetal demise (IUFD).

Methods: Intertwin discordance in the CRL and NT was calculated as a percentage of the larger CRL and NT, respectively. The performance of inter twin discordance (CRL ≥ 10% and NT≥ 20%) for predicting complications was analysed using standard statistical screening test methods.

Results: Fifty-eight MCDA twin pregnancies were studied. Out of them, 19 (32%) pregnancies resulted in one of the complications studied (4 TTTS, 10 sFGR, 5 IUFD). CRL and NT discordance showed an increased probability of developing complications positive likelihood ratio (LR+) {95% confidence interval}: 2.05 {0.46-9.23} and 1.88 {1.03-3.45}, respectively. NT discordance showed a sensitivity of 57%.

Conclusions: 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.