SD is the ratio of systolic to diastolic pressure in the umbilical artery of the fetus, and is usually written in the format S/D. The normal range of umbilical artery blood flow at 28 weeks of pregnancy, including other pregnancy periods, is 2.5-3.0. Generally, an S/D value ≥3.0 is considered abnormal, which is commonly seen in umbilical cord entanglement, torsion, kinking, or shortening, suggesting that there is a high circulatory resistance in the placenta, and a decrease in the placental perfusion, resulting in a state of nutritional deficiencies and hypoxia in the fetus, which may lead to the development of fetal growth restriction. If the S/D value is slightly high in the middle of pregnancy, you can observe the fetal movement first and review it in a few days, and some of them are physiological and not necessarily absolute abnormalities. If the amniotic fluid volume decreases and fetal movements are too slow or too frequent, intrauterine hypoxia is more likely and requires active treatment. Late pregnancy is more significant than mid-pregnancy. When the S/D value exceeds 3.0, it is necessary to do fetal heart rate monitoring and biophysical scoring, etc. for evaluation, and if necessary, give oxygen and apply medications according to the doctor’s instructions, to avoid intrauterine distress, which can lead to death in serious cases.