Umbilical blood flow is one of the indicators of fetal blood supply. Fetal umbilical blood flow monitoring is a non-invasive test. It can be detected using ultrasound or a specialized fetal umbilical blood flow monitor can be used. Fetal umbilical blood flow monitoring is a commonly used prenatal test to find out if the fetus is showing signs of hypoxia in the uterine cavity. The umbilical blood flow ratio S/D, which is the ratio of systolic to diastolic umbilical cord blood flow, has its corresponding reference ranges at different periods of pregnancy, and should be less than 3 in late pregnancy. With the increase of gestational weeks, placental circulatory resistance decreases, blood flow increases, and its blood flow S/D ratio decreases compared with the previous one, suggesting that the fetal-placental blood supply is sufficient to meet the needs of fetal growth and development. On the contrary, when the placental circulatory resistance increases, the S/D value of umbilical blood flow increases, suggesting that the fetus may have intrauterine ischemia and hypoxia, which is mostly considered to be caused by gestational hypertension or umbilical cord factors, such as umbilical cord kinking or umbilical cord torsion, etc. Proactive interventions should be given, such as lying on the left side of the bed, oxygen intake, and medication when necessary, or it may result in intrauterine hypoxia and other adverse outcomes. Umbilical blood flow ratio, S/D, is one of the most important indicators of fetal placental function. If this value is abnormal, we should contact the doctor for active intervention, otherwise, intrauterine distress or even intrauterine fetal death may occur and other adverse outcomes of pregnancy.