The umbilical artery S/D value, which can reflect the altered placental hemodynamics, is used to evaluate the placental function. The normal S/D value at 33 weeks of gestation should be less than 3. If it is greater than 3, it indicates the possible presence of intrauterine hypoxia. The S/D value is usually measured at 20 weeks of gestation and approaches 5 at 20 weeks of gestation, and decreases gradually with the increase of gestational weeks to below 3 after 30 weeks of gestation. If the S/D value is greater than 3 at 33 weeks of pregnancy, it indicates that the placental cord function is declining, and long-term non-improvement cannot exclude the possibility of fetal intrauterine hypoxia, so the pregnant woman can adopt the left lateral position to inhale oxygen, and at the same time give fetal heart monitoring and ultrasound examination to dynamically observe the fetal intrauterine growth. If the S/D value is abnormal once in a while, the pregnant woman can recheck the S/D ratio at an interval of 3-7 days and monitor the fetal movement closely during that period. If abnormalities occur, it is important to seek medical attention in time to investigate whether there are problems such as umbilical cord compression and placental function degeneration. If the S/D value is back to normal and the fetal heart monitoring is not abnormal, regular maternity checkups are sufficient.