A 30-years-old Taiwanese female in her second pregnancy spontaneously conceived a monochorionic twin pregnancy. A routine ultrasound at 27 weeks of gestation revealed a selective intrauterine growth restriction (sIUGR) fetus and an appropriate gestational age (AGA) fetus. The AGA fetus was found to have a fetal intra-abdominal umbilical vein (FIUV) varix. Serial ultrasounds showed no changes in the FIUV varix. 2 weeks later, the pregnancy progressed to twin–twin transfusion syndrome (TTTS). Repeated amnioreductions were required at 29 and 30 weeks gestation. The babies were delivered by caesarean section at 31 weeks due to fetal distress in the sIUGR fetus. Both fetuses survived the neonatal period with problems of prematurity. The FIUV varix disappeared a few days after delivery.