The normal range of ultrasound S/D values is between 1.5 and 3. S/D is the ratio of the velocity of the blood flow in the umbilical vein within the umbilical cord, since it is arterial blood that flows in the umbilical vein, that is, the ratio of the peak systolic flow rate to the minimum diastolic flow rate. In early pregnancy this ratio is slightly higher, closer to 3; in mid to late pregnancy the ratio is between 1.5 and 2.5. It is mainly used to evaluate whether the fetus is hypoxic in the uterus and whether distress syndrome is present. If the S/D value is persistently abnormal, too high or too low in early pregnancy, the internal diameter of the umbilical vessels is considered too thin, making the fetus lack of nutrients and leading to growth retardation. If the S/D value is abnormal in the middle and late stages of pregnancy, especially if it is often higher than 3, it is considered that the umbilical cord is too tightly wrapped around the neck, trunk and limbs, which affects the circulation of blood flow, and may lead to fetal hypoxia and distress syndrome, in which case oxygen should be administered, fetal heart rate should be monitored, close observation should be made, and caesarean section should be performed if necessary to bring forward the delivery of the baby.