Twin fetal transfusion syndrome and selective fetal growth restriction are two common complications of monochorionic twins that may occur at any gestational week, with most being detected in midtrimester. Therefore, once a diagnosis of monochorionic twin is made, biweekly ultrasound starting at 16 weeks is recommended to assess fetal size, amniotic fluid and umbilical artery blood flow for early detection of complications. Obstetrical examination and monitoring at a medical institution experienced in the diagnosis and management of twin fetuses is recommended. There are two criteria for diagnosing twin-to-twin transfusion syndrome (TTTS) during pregnancy: a single chorionic twin with excess amniotic fluid in one fetus and low amniotic fluid in the other fetus, meeting these two conditions will diagnose TTTS-stage 1; in severe cases, the smaller fetus may have loss of bladder (stage 2), present with extreme abnormalities of Doppler flow (stage 3), generalized edema and heart failure in the larger fetus (stage 4), and in the most severe cases, may lead to intrauterine death (stage 5). The pathology of twin fetus transfusion syndrome is mainly due to the disruption of hemodynamic balance caused by the traffic vessels between the two fetuses, while the weight of the two fetuses sometimes does not differ significantly. Twin mothers-to-be with twin-fetus transfusion syndrome are at high risk of premature rupture of membranes due to excessive amniotic fluid, and if left untreated, most of them go to miscarriage and immature preterm delivery with neonatal mortality up to 90-100%. Selective fetal growth restriction (sIUGR) refers to severe growth restriction in one fetus and normal development in the other fetus in monochorionic twins, but the amniotic fluid volume may be normal in both fetuses, and there is a difference in the assessment of cardiac function between the two fetuses, for example, TTTS tends to have a high cardiac burden in the recipient and is prone to abnormal cardiac function, this helps to differentiate the diagnosis from TTTS. In other words, sIUGR places more emphasis on the difference in fetal weight, and the cardiac and renal functions of the two fetuses are generally not significantly altered, so that significant differences in amniotic fluid volume between the two fetuses are less likely to occur during pregnancy. The pathological mechanism is mainly related to the small fetus with a small percentage of placental share and marginal attachment of the umbilical cord. There are three types of umbilical artery flow in small fetuses: type I with normal diastolic flow, type II with continuous loss or reversal of diastolic flow, and type III with intermittent loss or reversal of diastolic flow in the umbilical artery. The clinical prognosis of type I is better and is basically the same as that of uncomplicated monochorionic twins, so most of them only require close observation. Type II and III sIUGR may have sudden intrauterine death of small fetuses, large fetal blood loss, and adverse outcome of preterm delivery during the expectation process, therefore, their monitoring during pregnancy needs to be very close, and the indication and timing of termination of pregnancy are more difficult to grasp.