Uterine artery resistance is measured by ultrasonography. Parameters measured include resistance index (RI), pulsatility index (PI), and peak systolic flow rate/diastolic flow rate (S/D). The parameters vary at different times of the menstrual cycle. In the luteal phase (the period of fertilized egg implantation) after ovulation, the PI and RI values are lower compared to the follicular phase of development, and the blood flow in the uterine arteries increases, which is conducive to the implantation of a fertilized egg and the development of the embryo. Therefore, uterine artery blood flow is measured to assess the endometrial tolerance and to predict whether the fertilized egg will be able to settle and develop. The parameters of uterine artery blood flow will be different in different weeks of pregnancy: early pregnancy average PI: <2.25, RI: <0.75, S/D: <6; middle pregnancy average PI: <1.5, RI: <0.73, S/D: <3.6; late pregnancy average PI: <0.82, RI: <0.58, S/D : <2.6. Lower values of PI and RI indicate better ovarian and uterine blood perfusion, good endometrial tolerance and high embryo implantation rate. On the contrary, if the blood flow of uterine artery is abnormal, the resistance is too high, and the blood perfusion is poor, which leads to the failure of implantation of fertilized egg, embryo dysplasia, and miscarriage.