Recently, the group of Nicolaides reported on a large series (n = 512) of nuchal translucency discordance to predict TTTS [33] . An increased nuchal translucency is more common in monochorionic gestations and is 1 95th percentile in at least one twin in 13% of monochorionic pregnancies, suggesting an inf luence of the intertwin anastomoses and possible early intertwin transfusion imbalance [34] . Discordance in nuchal translucency of 20% or more ident i f ied 52% of cases compl icated by TTTS, but only 24% eventually developed TTTS [33] . This low positive predictive value for TTTS as a separate entity is in agreement with our findings [32] . However, in our series as well as in others [35–37] , nuchal translucency was not a significant predictor for TTTS. In our series, 45% of twins had nuchal discordance 6 20% in comparison with only 25% in the series by Nicolaides’ group. Also, in our analysis, we did not exclude TTTS cases with normal Doppler measurements. Nevertheless, discordance in crown-rump length was also a significant predictor of TTTS in the series by Nicolaides, which is in agreement with our findings [32] as well as another series [35] .
Inclusion of Doppler measurements, such as the ductus venosus pulsatility index, may allow for a better prediction of TTTS in the first trimester [38] . Another large series by the Nicolaides’ group (n = 179) [39] reported that similar to increased nuchal translucency, absent or reversed a-wave in the ductus venosus is more common in monochorionic twin gestations. As such, there is an abnormal ductus in at least one twin in 18% of pregnancies. Although the ductus venosus was a better predictor for TTTS than the nuchal translucency or crown-rump length, its sensitivity for TTTS was 40% with a positive predictive value of 30%. Similar to the series on nuchal translucency discordance, only TTTS cases with abnormal Doppler measurements were included in the analysis, whereas these constitute less than half of all TTTS cases. In a recent smaller series on 99 monochorionic twins more optimistic figures were reported for ductus venosus Doppler evaluation for the prediction of TTTS, with a sensitivity of 75% and a positive predictive value of 56%. In these latter series, TTTS was defined as oligohy-