Normal range of fetal blood flow parameters

In daily life, women often have to undergo various examinations during pregnancy. These examinations can also determine the baby's development and whether there are malformations or other congenital diseases. Among them, umbilical cord blood testing is relatively common. The fetal blood flow has a normal reference value range. As long as the umbilical cord blood flow test is normal, it means that the baby is developing well and the possibility of congenital malformations is very small, so this examination must be taken seriously.

In normal pregnancy, the S/D and RI values of the fetus in umbilical cord blood flow detection tend to decrease with the passage of pregnancy. In particular, the change in the S/D value is an important indicator for understanding whether the fetal development is normal. Stage 1: If the S/D and RI values increase between 26 and 28 weeks of pregnancy (S/D should be less than 3 and RI should be less than 0.8 after 28 weeks), the following should be considered: (1) Fetal malformation: Congenital fetal diseases are closely related to umbilical artery resistance and further ultrasound examination should be performed. (2) Umbilical cord abnormality: When the umbilical cord is entangled, too long or too short, or too thin, affecting the placental circulation, the blood flow impedance index will become abnormal. If the S/D value is higher than normal and ultrasound shows abnormal conditions such as the umbilical cord around the neck, the baby should be closely observed according to the stage of pregnancy.

(3) Placental dysfunction: Pathological changes in the placenta can lead to a decrease in placental volume, a decrease in the total cross-sectional area of effective blood vessels, increased blood flow resistance, and a decrease in blood perfusion. (4) Intrauterine growth retardation (IUGR): There are many reasons for pregnant women to experience IUGR. In addition to genetic nutrition, harmful contact, malformations, viruses and other factors, the proportion of IUGR caused by pregnancy appendages such as the placenta is increasing, which is manifested by increased S/D and RI values. Phase II: Monitoring after 36 to 37 weeks, the umbilical artery blood flow impedance is divided into three levels. Grade 1: S/D value 3.0, but 4.0 will lead to poor perinatal prognosis. The third stage: Umbilical artery impedance index during delivery: When a normal pregnant woman is about to give birth, the S/D value does not change significantly. If the index is abnormal, it indicates a poor perinatal prognosis. If abnormal umbilical cord blood flow is diagnosed, it is best to count fetal movements every day to monitor the fetal condition. If the abnormal umbilical cord blood flow is not very serious, it is recommended to lie on the left side. If there is no improvement, oxygen therapy is required if necessary. Supplementary explanation: The ratio of the maximum blood flow velocity during systole (S) to the blood flow velocity during end diastole (D) of the umbilical artery (S/D, A/B). By observing the dynamic changes of the S/D (A/B) ratio, the development of the placenta can be judged.

Under normal circumstances, the placenta gradually increases in size as the fetus develops. Vascular resistance gradually decreases, and the S/D (A/B) ratio gradually decreases. From 25 to 41 weeks of pregnancy, the S/D ratio decreased almost linearly from 2.8 to 2.2. The fetus has intrauterine growth retardation, the placenta and fetus are poorly developed, the placenta does not increase in size, and vascular resistance does not decrease. Therefore, the S/D ratio does not decrease. Clinically, if S/D does not decrease regularly or even increases, it indicates that the fetus is maldeveloped. In normal pregnancy, the S/D ratio of the ductus venosus decreases with increasing gestational age, from about 3 at 14 weeks of gestation to about 2 at 42 weeks of gestation. The S/D ratio of the ductus venosus is related to fetal hypoxemia. The umbilical artery value (umbilical artery blood flow impedance) reflects the oxygen delivery in the placenta. If this value rises abnormally, the main consequence is fetal intrauterine hypoxia.

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