S/D value is usually an indicator to assess whether the fetus is hypoxic in late pregnancy, and it tends to decrease with the increase of gestational weeks, generally, the S/D value is about 3.4 at 26 weeks of pregnancy, less than 3 after 30 weeks of pregnancy, and about 2.2 at 38 weeks of pregnancy. If the S/D value exceeds the normal range, the fetus is considered to be in hypoxia, which is mainly related to factors such as reduced fetal heart function, umbilical cord abnormality, placental dysfunction and gestational hypertension. 1. reduced fetal heart function: some fetuses with congenital heart disease can have reduced heart function, resulting in reduced circulatory function, resulting in increased S/D value; 2. umbilical cord abnormality: if the fetus has an umbilical cord that is tangled, If the umbilical cord is tangled, too long, too short, too thin, etc., the blood flow will be mechanically obstructed, which will affect the placental circulation and lead to an increase in S/D value; 3, placental dysfunction: pathological changes such as aging of the placenta can lead to a decrease in placental volume and a decrease in placental blood perfusion, making the placenta ischemic and hypoxic, leading to an increase in S/D value; 4, gestational hypertension: if a pregnant woman is combined with gestational hypertension, it can lead to vascular spasm and an increase in blood flow The S/D value will increase as a result of increased blood flow resistance. Therefore, the abnormal increase of S/D value may lead to serious consequences for the fetus in the uterus, which should be judged together with fetal heartbeat and fetal movement.