We investigated the umbilical vein volume flow (UVVF) at 11-14 weeks in monochorionic-diamniotic (MCDA) twins, focusing on the occurrence of complications. We considered 87 MCDA pregnancies. We used "E-flow" to detect the umbilical vein and measured the mean velocities and diameters of veins, also calculating the UVVF. Pregnancies were divided into four groups: a (twin-to-twin transfusion, TTTS); b (selective intrauterine growth restriction, sIUGR); c (discordance of fluid, DF); and d (with "normal" outcome). The main outcome of the study was the assessment of the difference of UVVF between twin 1 and twin 2 (larger and smaller twin). In eight of eight pairs of group a and seven of seven pairs of group b, the UVVF of fetuses 1 were significantly different than fetuses 2 (P<0.05). The UVVFs of fetuses 2 of group a were significantly lower in respect to the UVVFs of fetuses 2 of group d (P<0.05). We provide evidence of an important difference in UVVF in pairs that successively developed TTTS or sIUGR. The smaller fetuses of group a (TTTS) showed a lower venous return than fetuses 2 of group d (normal). First-trimester UVVF is lower in fetuses with a smaller crown-rump length in twin pairs at risk of TTTS or IUGR.