S/D is the ratio of the maximum end-systolic flow velocity (S) to the maximum end-diastolic flow velocity (D) of the umbilical artery detected using color Doppler ultrasound. s reflects blood flow and D reflects placental vascular resistance, and the S/D ratio is usually used as an index to assess placental vascular resistance, and is generally used to assess whether the fetal placenta is developing normally by S/D in late pregnancy. It can reflect whether the fetus is hypoxic in the mother’s body, i.e. whether fetal distress is occurring. If the S/D ratio is greater than 3, it indicates that the placental vascular resistance is elevated or the fetal heart function is reduced, which may lead to abnormal fetal circulation and fetal hypoxia, and the larger the ratio, the more serious the fetal hypoxia and the higher the risk, which may cause The higher the S/D ratio, the more serious the fetal hypoxia and the higher the risk, which can cause intrauterine growth retardation, and in serious cases, fetal brain dysplasia, developmental malformation and fetal death. Therefore, when an elevated S/D ratio is detected, if the degree is mild, the pregnant woman can adopt a left lateral position in order to increase the blood flow to the placenta and uterus, thus improving the intrauterine hypoxia of the fetus. If the degree is more severe, the pregnant woman can take oxygen under the guidance of the doctor and perform fetal heart monitoring at the same time. If the condition does not improve after the above measures, the fetus needs to be promptly admitted to the hospital to solve the problem of intrauterine hypoxia through other treatment methods in order to prevent serious consequences.