What is the normal S/D value of umbilical artery blood flow at different stages?

The umbilical artery blood flow S/D value is an important indicator that can be used to judge whether the placenta functions well and whether the fetus develops normally. During pregnancy, it is necessary to pay close attention to changes in the umbilical artery blood flow S/D value, because once the umbilical artery blood flow S/D value is abnormal, it means that the fetus is also abnormal and needs to be treated in time to ensure that the fetus has a good development in the mother's body. Therefore, it is very necessary for pregnant women to know what the normal S/D value of umbilical artery blood flow is. Generally speaking, due to different stages of pregnancy, the positive output value of the umbilical artery blood flow S/D value is also different. Here we will introduce you to the relevant knowledge of the umbilical artery blood flow S/D value, hoping that it will be helpful to everyone!

During normal pregnancy, the S/D and RI values ​​of the fetus tend to decrease as the pregnancy progresses. 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 of pregnancy), the following should be considered:

(1) Fetal malformation: Fetal congenital diseases are closely related to umbilical artery resistance and should be further examined by B-ultrasound.

(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, and an increase in blood flow resistance, resulting in a decrease in blood perfusion.

(4) Intrauterine growth retardation (IUGR): There are many reasons for pregnant women to suffer from IUGR. In addition to genetic nutrition, harmful contact, malformation, 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.

The second stage: monitoring after 36 to 37 weeks, the umbilical artery blood flow impedance is divided into three levels.

Level 1: S/D value <3.0, umbilical artery blood flow impedance is at a normal level.

Grade 2: S/D value > 3.0 but < 4.0, which will not cause acute fetal distress and should be treated promptly to prevent the condition from worsening.

Level 3: S/D value>4.0, which will lead to poor perinatal prognosis.

Stage 3: Umbilical artery impedance index during labor:

When a normal pregnant woman is about to give birth, there is no obvious change in the S/D value. If the indicator 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.

The ratio of the maximum blood flow velocity during systole (S) to the blood flow velocity during end diastole (D) of the umbilical artery blood flow (S/D, A/B) was used to observe

The dynamic changes of the S/D (A/B) ratio can be used to determine the development of the placenta. Under normal circumstances, as the fetus develops, the placenta gradually increases in size, 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 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 associated with 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.