The standard value of fetal umbilical artery S/D varies for different gestational weeks and is usually negligible. However, for late pregnancy the standard needs to be kept in mind that the umbilical vascular artery S/D value should be less than 3 after 28 weeks of gestation, the smaller the better. If the S/D is relatively high in late pregnancy, we need to pay attention to the umbilical vessels and umbilical blood flow problems and find the cause, whether it is attributed to the placental blood supply problem or the fetus itself, or to the pregnant woman. Sometimes the maternal blood pressure becomes higher and higher in the middle and late stages of pregnancy, which can affect the fetal S/D value, and problems with the maternal coagulation system can also affect the umbilical blood flow. In short, the S/D value of umbilical cord blood flow needs to be paid attention to from 28 weeks, if it is high, we need to find the cause and deal with the cause, if there is no problem, it can be less than 3.