The ratio of the maximum systolic blood flow velocity (S) to the end diastolic blood flow velocity (D) in the umbilical artery (S/D) and the dynamic changes of the S/D (A/B) ratio can determine the development of the placenta. Under normal conditions, the placenta gradually increases in size as the fetus develops. The vascular resistance gradually decreases, and the S/D ratio gradually decreases. From 25 to 41 weeks of gestation, the S/D ratio decreases almost linearly from 2.8 to 2.2. If the intrauterine development of the fetus is delayed, the placenta and the fetus are poorly developed, the placenta does not increase in size and the vascular resistance does not decrease. Therefore, the S/D ratio does not decrease. Clinically S/D does not decrease in a regular manner or instead increases, indicating fetal hypoplasia. In normal pregnancy, the S/D ratio of the venous catheter decreases with increasing gestational weeks, from about 3 at 14 gestational weeks to about 2 at 42 gestational weeks The S/D ratio of the venous catheter is associated with fetal hypoxemia. The umbilical artery value (umbilical artery blood flow impedance) reflects the delivery of oxygen in the placenta. The most important consequence of an abnormally high value is intrauterine hypoxia. The S/D and RI values of the fetus in a normal pregnancy tend to decrease as the pregnancy progresses, especially the change in S/D value is an important indicator to know whether the fetus is developing normally. The first stage: if the S/D and RI values increase from 26 to 28 weeks of pregnancy (S/D should be less than 3 and RI should be less than 0.8 after 28 weeks), we should mainly consider: (1) fetal malformation: fetal congenital diseases are closely related to umbilical artery resistance and should be further examined by ultrasound. (2)Umbilical cord anomaly:When the umbilical cord is entangled, too long or too short or too thin to affect the placental circulation, the abnormality appears as the blood flow impedance index. If the S/D value is higher than normal and the ultrasound shows abnormalities such as cord winding, close observation should be made according to the stages of pregnancy. (3) Placental dysfunction:Pathological changes of the placenta can lead to a decrease in placental volume, a decrease in the total effective vascular cross-sectional area, and an increase in blood flow resistance, resulting in a decrease in its blood perfusion. (4) Intrauterine growth retardation (IUGR): There are many causes of IUGR in pregnant women. In addition to genetic nutrition, harmful exposure, malformations, viruses and other factors, IUGR due to placenta and other pregnancy appendages is taking up an increasing proportion, manifesting as increased S/D and RI values. The second stage: monitored after 36-37 weeks, the umbilical artery blood flow impedance is divided into three levels. Grade 1:S/D value <3.0, umbilical artery flow impedance is at normal level. Grade 2:S/D value >3.0, but <4.0, which will not cause acute fetal distress and should be treated promptly to prevent deterioration of the condition. Grade 3:S/D value >4.0 will lead to poor prognosis of the perinatal infant. Stage 3: Umbilical artery impedance index during delivery: There is no significant change in S/D value in normal pregnant women at the time of delivery, if the index is abnormal, it indicates poor prognosis of the perinatal baby. If the diagnosis of abnormal umbilical blood flow is made, it is advisable to count fetal movements daily to monitor the fetal condition. If the abnormal umbilical blood flow is not very serious, it is recommended to adopt the left lateral position, and if it does not improve, oxygen therapy is necessary.