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Ductus venosus in twins: science matters

It was with great interest that we read the recently published guidelines on the role of ultrasound in twin pregnancy1. Although the content is in many ways commendable, the disregard of previous pivotal publications relating to the topic is disappointing. On Page 251, when the authors discuss the implications of discordance in nuchal translucency thickness (NT) and crown–rump length (CRL), they ignore the clinically relevant contribution of abnormal flow in the ductus venosus (DV) in screening for twin-to-twin transfusion syndrome (TTTS). We know from the papers of Sebire et al.2,3 that a higher prevalence of increased NT was noted among monochorionic (MC) twins along with a four-fold increase in the risk of developing TTTS in this subgroup. An intertwin difference in NT of more than 20% was a stronger marker for TTTS, but its sensitivity was unsatisfactory (around 50%)4. From our published peer-reviewed work, we could clearly demonstrate that abnormal DV blood flow in at least one fetus, used as the only marker, was associated with a relative risk for developing TTTS of 11.86, with a sensitivity of 75.0% and a specificity of 92.0%5. If we further consider the combination of NT and DV blood flow in twin pregnancies with non-discrepant NT but abnormal DV blood flow in at least one of the fetuses, the risk of developing TTTS was found to be increased by a factor of 10, and for those presenting with discrepant NT ≥ 0.6 mm along with abnormal DV blood flow, the risk of developing TTTS was 21 times higher5. These conclusions were based on evaluation of 99 MC twin pairs (a not inconsiderable number), in good agreement with the results of Maiz et al.6 from 179 MC pairs. We believe that the role of the DV in screening for TTTS in MC twin pregnancy cannot be overlooked, since it is clinically relevant and may contribute to the improvement of obstetric management. The emphasis given in the published guidelines1 to discrepant CRL in screening for TTTS seems exaggerated. Discrepancy in CRL between twins was only indicative of a high risk of developing TTTS in those with a difference ≥ 10 mm7. However, CRL discrepancies of this magnitude are rarely found in clinical practice, leading to a low sensitivity (8.3%) when applied to our study population. Furthermore, we have shown that, in twin pregnancies with abnormal DV flow in at least one fetus, the median discordance in birth weight was greater than it was in those with normal DV flow in both twins (13.2% vs 7.8%, P = 0.006)8. We fully support that evidence-based guidelines should be produced by experts. However, we cannot peacefully accept that peer-reviewed, clinically relevant work that is easily accessible should be overlooked. It is unfortunate that politics is frequently a trigger for publication, but we believe that science still matters and should prevail.