In late pregnancy, that is, after 28 weeks, the normal value of PI is 0.95±0.19 and the normal range of RI value is less than 0.63±0.07. Umbilical blood flow PI and RI are two different blood flow spectra, in which PI refers to the pulsatility index and RI refers to the umbilical artery flow resistance index, and the normal values of the two indexes are not the same. 1. PI: in non-pregnancy ultrasound suggests that the PI is generally less than 1.2 for the level of normal values, after 8 weeks of pregnancy is maintained at 0.8 or so, after 28 weeks of pregnancy, the PI is normal value of 0.95 ± 0.19, if the PI value increases, suggesting that there may be intrauterine hypoxia. 2. RI: the normal range of RI value after 28 weeks of pregnancy is less than 0.63±0.07, if the RI value is elevated, consider whether there is a fetal umbilical cord around the neck, intrauterine fetal developmental delay, placental dysfunction, umbilical cord anomalies, fetal anomalies and other conditions. If abnormal umbilical artery PI and RI are found by ultrasound, intrauterine hypoxia, fetal developmental abnormality, placental dysfunction cannot be excluded, and prompt treatment should be given according to medical advice.