Ultrasound S/D is the velocity ratio of blood flow in the umbilical cord, which is the ratio of the peak systolic flow rate to the lowest diastolic flow rate, with the normal value not exceeding 3. In early pregnancy, the ratio is slightly higher, close to 3, and in mid to late pregnancy, the ratio is usually between 1.5 and 2.5, mainly to evaluate whether the fetus is hypoxic in utero and whether distress syndrome is present. When ultrasound is done to examine the fetus, it is important to routinely measure the flow rate of the umbilical vein, since arterial blood flows in the umbilical vein, and then calculate the ratio. If it stays high and often exceeds 3, consider that the umbilical cord is too thin or too short, the cord is wrapped around the neck too tightly or the cord is wrapped around the limb too tightly. If it appears in early and mid pregnancy, it will lead to fetal growth retardation. If it occurs in late pregnancy, it will lead to intrauterine hypoxia, and measures should be taken, such as oxygen, fetal heart monitoring and close observation, and if necessary, cesarean delivery.