The umbilical cord is the link between the mother and the fetus, providing nutrients for fetal growth and development, and transporting fetal metabolites to the mother and out of the body. In clinical practice, the spectral waveform of the umbilical artery is commonly used to determine the state of hypoxia. The first change in hypoxia is a decrease in end-diastolic blood flow and an increase in S/D, RI and PI values. When hypoxia enters the late stage of redistribution, there may be a gradual decrease in end-diastolic flow, absence of end-diastolic flow or even absence of the entire diastolic flow. Retrograde umbilical artery flow and inversion of diastolic flow occur during severe hypoxic loss of compensation. Systolic to diastolic ratio (S/D) = peak systolic flow rate (S) / end-diastolic flow rate (D) Resistance index (RI) = [peak systolic flow rate (S) – end-diastolic flow rate (D)] / peak systolic flow rate (S) Pulsatility index (PI) = [peak systolic flow rate (S) – end-diastolic flow rate (D)] / time averaged flow rate (M) All the above three indices are resistance indices All of them increase with increasing resistance, however, the three meanings are different. In response to high resistance, S/D only utilizes peak systolic flow and end-diastolic minimum flow, but does not reflect the flow of the whole cycle; PI reflects the resistance of the whole cardiac cycle; RI only utilizes peak systolic flow and end-diastolic minimum flow, but also reflects the presence of end-diastolic flow and the presence of regurgitant blood, when end-diastolic flow disappears or regurgitant flow exists The RI value is equal to 1 or greater than 1. Currently, many international prenatal diagnosis centers use PI to measure and evaluate fetal hemodynamics, especially the umbilical artery. The fetal umbilical artery flow spectrum depends on the following factors: fetal myocardial contractility, vessel wall elasticity and blood viscosity, umbilical-placental circulatory resistance, fetal heart rate and umbilical cord sampling site. Fetal growth is influenced by the flow, oxygen and nutrient content of umbilical blood, and the S/D value of umbilical artery has become an important indicator for evaluating placental function and fetal development, and maternal factors such as hypertensive disorders during pregnancy, gestational diabetes and other obstetric complications, and fetal factors such as placental hemangioma, fetal malformation and FGR can lead to abnormal S/D values of umbilical artery. The umbilical artery S/D value can be abnormal. From 16 weeks of gestation, the umbilical artery flow velocity wave returns to baseline in the diastolic phase, and as the placental vascular resistance gradually decreases with increasing weeks of gestation, the diastolic flow in the umbilical artery increases, and the S/D value gradually decreases, more rapidly before 24 weeks, ≤4 before 24 weeks, and then gradually decreases to <3 after 30 weeks. If the S/D value is greater than 3, the positive rate of FGR is 49%. It also reflects intrauterine hypoxia, which puts the fetus at high risk of intrauterine death. When the umbilical cord is too tightly wound and severely compressed, it may also lead to obstruction of umbilical artery blood flow and increase the S/D value. Abnormal blood flow in the umbilical artery is a state, not a disease, so there is no definite and effective treatment at home or abroad. In my opinion, we need to actively search for and treat the primary cause of the disease and treat the cause. For example, antispasmodic treatment for hypertensive disorders in pregnancy and blood pressure control; rehydration therapy for increased blood viscosity can play a diluting role; intermittent oxygenation for mothers with decreased placental function; behavior modification, including maternal smoking cessation and adjustment of work and rest time; and specific therapies, including maternal low-dose aspirin, whose efficacy has not been recognized. When abnormal umbilical artery blood flow is detected, monitoring should be intensified to take advantage of the timing of termination of pregnancy and prevent fetal accidents. When the S/D value is greater than 4 or the RI value is greater than or equal to 1 (presence of diastolic flow deficit or regurgitation) in late pregnancy, prompt termination of pregnancy is required. There are also foreign data showing that even if there is loss of umbilical artery blood flow or reflux, the fetus still has a safe period of a few days to a few weeks, and for those who are considered to have immature fetal lungs, there is still time to promote fetal lung maturation, during which the patient's family needs to fully understand the condition and strengthen fetal monitoring.