Haemoglobin discordances in twins: is "really" due to differences in timing of cord clamping? A consideration to Verbeek L and co-authors

Dear Editor, We read with interest the article by Verbeek L. et al [1], showing that the second-born twin has higher levels of hemoglobin (Hb) than first-born co-twins after vaginal delivery (VD; Hb differential effect does not occur in twins delivered by Caesarean section. Since Hb difference is present in both uncomplicated monochorionic (MC) and dichorionic (DC) twin pairs, authors focused on the time difference of umbilical cord clampings (UCC) for the two twins, rather than vascular anastomoses (absent in DC twins). Precise timing data unfortunately were not recorded. However, beside UCC timing, other factors should be taken into account for the VD management. Recent observations have provided compelling evidence demonstrating that UCC timing is not the only determinant of net placental -to-infant blood transfusion [2]; uterine contractions and lung aeration result to be determinant factors influencing umbilical artery and venous blood flows[2]. Specifically the uterine contractions during the third stage of labor significantly increase the placental-to-neonatal gradient and may facilitate 50% of placental transfusion [3]; such effect is also reported in single term neonates when the "two step" head-to-body delivery method is used [4]. In our opinion, uterine contractions can affect the placental transfusion more than UCC timing in the vaginally born twins: the second-born twin is exposed to the contractions that lead to the birth of the first twin! These additional contractions can increase the placental transfusion and the risk of polycithemia both in DC and MC twins; moreover, in second-born MC twin, contractions can determine acute inter-twin blood transfusion through placental vascular anastomoses. In agreement with authors [1], targeted studies in the twins delivered vaginally should be carried out to establish the optimal UCC timing; anyway we recommend evaluating also the effect of uterine contractions as well as medications administered to the mothers, such as oxytocin-like components.