Abstract: Objective: To investigate the relationship between the uterine and endometrial blood flow index and the pregnancy outcomes of frozen-thawed embryo transfer (FET) cycles. Methods：Three hundred and ten women who had undergone FET were grouped into two groups, the pregnancy group (n=153) and the non-pregnancy group (n=157), according to the clinical outcomes. All patients were asked to monitor the endometrial thickness, endometrial volume(V), endometrial and subendometrial blood flow on the endometrial transformation days and before transplantation day. Results：The uterine artery pulsatility index(ＰＩ), the peak systolic velocity(PSV)/end diastolic velocity(EDV) on the endometrial transformation day in the pregnant group were significantly lower than those in the non-pregnant group(P＜0.05). The PI and resistance index (RI) on the transplantation day in the pregnant group was significantly lower than those in the non-pregnant group, while the flow index(FI) and V value in the pregnant group were significantly higher(P＜0.05). The area under the ROC curve of endometrial volume in the pregnancy group was significantly higher (0.637, P＜0.01). The uterine artery PSV, EDV and RI on the pre-transplant day were lower than those on the transformation day, while FI and V value on the pre-transplant day were higher than those on the transformation day(P＜0.05). The correlation coefficient between the uterine artery RI, PI, V and endometrial thickness were -0.431, -0.775 and 0.796(P＜0.01). The pregnancy rate in those women with the uterine RI 0.70-0.85 was significantly higher than that in those women with the uterine RI 0.86-0.95 (53.18% vs. 40.00%, P＜0.05). Conclusions：Those women with lower RI and PI on the transplantation day are more likely to get pregnancy. The V value is helpful for predictiving the endometrial receptivity. The PI and RI may affect the endometrial receptivity.

Key words: Ultrasonography, Doppler, color, Embryo transfer, Pregnancy outcome, Hemodynamics, Endometrium