Smithson et al.1 described a case of early-onset growth discordancy in a monochorionic twin pregnancy, highlighting one of the prenatal management issues which may arise in such high risk conceptions. Discordant fetal size in the first trimester is, in fact, quite well reported in both monochorionic and dichorionic twin pregnancies. In a study of 485 twin pregnancies resulting in live births, there was no significant difference between monochorionic and dichorionic pregnancies in intertwin crown–rump length disparity at 10–14 weeks of gestation (median [range]: 4.3% [0–18.8%] and 3.4% [0–25.5%], respectively)2. Furthermore, in a series of 132 monochorionic twin pregnancies examined at 10–14 weeks of gestation, the median intertwin difference in crown–rump length was 3 mm but ongoing pregnancies with crown–rump length discordancies of up to 16 mm were observed3. The finding of first trimester discordancy for crown–rump length was not associated with an increased risk of severe twin-to-twin transfusion syndrome; the greatest risk for this complication being in cases with similar fetal crown–rump length but discordant fetal nuchal translucency thickness3.
The pathophysiological basis explaining the development of discordant first trimester crown–rump length and nuchal translucency in monochorionic twins, and the relationship of these parameters to future development of severe twin-to-twin transfusion syndrome, is probably related to the number and type of interfetal placental anastomoses, which changes dynamically throughout the first half of gestation4. Pregnancies with discordant crown–rump length but similar fetal nuchal translucency are unlikely to have a large number of asymmetrical placental vascular connections present, therefore tend to remain discordant for fetal size and amniotic fluid volume throughout pregnancy but without development of polyhydramnios-associated classical severe twin-to-twin transfusion syndrome requiring in utero treatment4. Extensive discussion of these possible mechanisms, and the findings that first trimester sonographic features do not definitively predict subsequent development of severe twin-to-twin transfusion syndrome, can be found in a recent publication on this topic4.
Smithson et al.1 have highlighted that first trimester discordancy for fetal size may occur in monochorionic twin pregnancies. In all such cases, in addition to fetal crown–rump length, it is important to assess other parameters which may reflect the underlying placental vascular pattern, such as fetal nuchal translucency measurements and Doppler velocimetry of the ductus venosus5, in order that the most accurate assessment of risk for development of subsequent severe twin-to-twin transfusion syndrome can be provided, and hence, appropriate antenatal surveillance can be planned.