The umbilical artery SD value refers to the ratio of fetal umbilical artery systolic pressure to diastolic pressure, which is related to fetal blood supply. The S/D value of umbilical artery blood flow is evaluated for placental function by measuring the ratio of peak systolic (S) to peak diastolic (D) of the umbilical artery in late pregnancy. As the gestational weeks increase, the S/D value gradually decreases, and the umbilical artery S/D value can reflect changes in placental hemodynamics. A normal late pregnancy S/D value of 3, if an abnormal S/D value is found, it should be treated promptly.
When the placental function is poor or the umbilical cord is abnormal, this ratio will be abnormal. In normal pregnancy, as the gestational age increases, the fetus needs to increase the S to decrease and the D to increase, causing the ratio to decrease. In near term pregnancy, the S/D is less than 3. Among them, S represents the peak systolic flow velocity, reflecting blood flow, and D represents the end diastolic flow velocity, reflecting placental vascular resistance.
If diagnosed with abnormal umbilical blood flow, it is best to monitor fetal condition by counting fetal movements every day. If the abnormal condition of umbilical blood flow is not very serious, it is recommended to adopt a left lateral position. If there is no improvement, oxygen therapy may be necessary.
The ratio of the maximum systolic blood flow velocity (S) to the end diastolic blood flow velocity (D) of the umbilical artery (S/D, A/B), and the dynamic changes in the S/D (A/B) ratio can be observed to determine the development of the placenta. Under normal circumstances, as the fetus develops, the placenta gradually enlarges. The vascular resistance gradually decreases, and the S/D (A/B) ratio gradually decreases. The S/D ratio almost linearly decreased from 2.8 to 2.2 from 25 to 41 weeks of pregnancy. Fetal intrauterine growth retardation, placental and fetal dysplasia, no enlargement of placenta, and no decrease in vascular resistance. Therefore, the S/D ratio does not decrease. In clinical practice, if the S/D ratio does not decrease regularly or instead increases, it indicates poor fetal development.
During normal pregnancy, the S/D ratio of the venous catheter decreases with increasing gestational age, from around 3 at 14 weeks to around 2 at 42 weeks. The S/D ratio of venous catheters is associated with fetal hypoxemia. The umbilical artery value (umbilical artery blood flow impedance) reflects the transport of oxygen in the placenta. If this value abnormally increases, the main consequence is causing intrauterine hypoxia in the fetus.
