Fused yolk sacs in a case of forked umbilical cord in monochorionic diamniotic twin pregnancy: sonoembryological implications

Abstract

Forked cord (FC), which presents as a fused insertion of two umbilical cords into a monochorionic placenta, is a rare entity, especially in monochorionic diamniotic (MCDA) twin pregnancies. Herein, we describe a case of a MCDA pregnancy with FC, which showed characteristic findings on first-trimester ultrasound examination. A healthy 29-year-old nulliparous woman presented at 8 weeks of gestation following a positive result on pregnancy self-testing. Transvaginal ultrasound revealed two fetuses with active heartbeats and both had their own amniotic membrane, suggesting a MCDA pregnancy (Figure 1a). Noteworthy was that the two yolk sacs were completely fused (Figure 1b) and the connecting stalks, which are the site of the future umbilicus, were located in close proximity to one another (Figure 1c). On transabdominal ultrasound screening at 16 weeks of gestation (Figure 2a), FC was diagnosed but no other abnormalities were detected. The pregnancy course was uneventful and healthy female babies, weighing 2285 g and 2345 g, were born vaginally at 36 weeks. The umbilical and placental inspection revealed that each umbilical cord had two arteries and one vein, which fused near the placental surface, where the amniotic membranes divided the amniotic cavities (Figure 2b). Most cases of previously reported FC were observed in monochorionic monoamniotic (MCMA) twin pregnancies, which were frequently accompanied by an acardiac twin1, 2. Thomas et al. was the first to report an antenatally diagnosed case of FC in an otherwise uncomplicated MCDA pregnancy, in which FC was recognized at 30 weeks, and healthy infants (weighing 2460 g and 2490 g) were delivered at 35 weeks3. Frisch et al. reported a case of FC in a MCMA pregnancy diagnosed at 10 weeks, in which the twin pair (weighing 1169 g and 935 g) was delivered at 29 weeks because of abnormal fetal heart rate patterns4. In our case, which is the third report in the literature of liveborn twins with prenatal diagnosis of FC, the unique findings were the fused yolk sacs, with the amniotic membranes dividing the two fetuses as early as 8 weeks of gestation, in addition to the closely located connecting stalks. From an embryological viewpoint, the folding of the embryo and the expansion of the amniotic cavity, which bring the connecting stalk and yolk sac together to form the umbilical cord, occur after 9 weeks of gestation5. We postulate that our findings at 8 weeks of the fused yolk sacs and the closely located connecting stalks might be forecasting subsequent FC, even when the amniotic membranes dividing two embryos are visualized. As for the blood volume balance between the twins, we agree with the speculation of Thomas et al. and Frisch et al. that a fenestration between the two umbilical vein walls may have had a protective effect on the hemodynamic status of the twins3 and that FC itself in MCMA and MCDA pregnancies does not necessarily cause adverse perinatal outcome4. Our first-trimester findings provide new insights into sonoembryology, especially in relation to umbilical cord development from connecting stalks.