Fig. 1

Classifications of type 1, type 2, and type 3 selective intrauterine growth restriction (T1& T2&T3 sIUGR) in monochorionic twins via placental sharing, velamentous cord insertion, anastomoses, and umbilical artery flow. The white dotted lines connect the smallest diameter vascular anastomoses points on the surface of the chorionic plate and divide the placenta into larger and smaller twin portions. The red regions in the d, e, and f indicate that the blood flow of umbilical artery directs toward the ultrasound probe. The blue regions in the e, f show that the direction of blood flow in the umbilical artery is opposite to the ultrasound probe. a T1 sIUGR placenta - placenta was infused with placental vascular dye injection. This displays unequal placental sharing and discordance cord insertion as the features of sIUGR placenta, but vascular anastomoses remain similar to uncomplicated monochorionic twins. b T2 sIUGR placenta - large arterioarterial anastomoses (2 ≥ mm, yellow circle) are present in 18% of placenta. c T3 sIUGR placenta - large arterioarterial anastomoses (≥2 mm, yellow circle) are present in 98% of placenta. d In T1 sIUGR, the umbilical artery Doppler waveform has positive end-diastolic velocity in the umbilical artery. e In T2 sIUGR, there is absent or reversed end-diastolic flow (AREDF) in the umbilical artery. f In T3 sIGUR, there is a cyclical or intermittent pattern of AREDF in the umbilical artery