The normal value of umbilical artery blood flow spectrum is usually no more than 3 in late pregnancy, but this value is not fixed and will gradually decrease with the weeks of pregnancy. Umbilical artery blood flow spectrum refers to the changes in umbilical artery blood flow during systole and diastole detected during fetal ultrasonography, which is used to determine whether there is any fetal abnormality in the uterus, and is usually reported clinically by the S/D ratio. However, the normal value of S/D is not fixed. At 36 weeks of pregnancy, the upper limit of normal value is 3, and it will be less than 3 after 37 weeks. The upper limit of normal value at week 37 is 2.9, and the upper limit of normal value at week 38 is 2.8, and the ratio decreases with the increase of gestational weeks. If the S/D ratio is higher than normal, it means that the fetus may have intrauterine hypoxia, and the pregnant woman should be vigilant and seek medical treatment promptly.