Background & Objective: Endometrial thickness and vascularity are commonly used predictors of endometrial receptivity. We aimed to evaluate whether the three-di-mensional ultrasound assessment of changes observed in the endometrium on the day of FET compared with that on the day of initiation of progesterone has an impact on the clinical pregnancy rates in FET cycles.   
Materials and Methods: A prospective study was per-formed in a specialized fertility center from February 2021 to February 2023. The study included 150 FET cycles in which endometrial preparation was done with hormonal replacement therapy. Using 3D ultrasound, the alterations in endometrial parameters (thickness, volume, and endometrial blood flow indices) among the day pro-gesterone was initiated and the day FET were evaluated were compared. An analysis was conducted on the rela-tionship among endometrial changes and clinical preg-nancy rates (CPR).    
Results: Overall, 161 participants were enrolled in the current study, 11 were excluded, and 150 were included into statistical analysis. Among those 150 patients, 102 were pregnant (68%) and 48 were not pregnant (32%).  Baseline demographic, clinical and cycle characteristics were matched between groups with no significant differ-ences detected. Clinical pregnancy rates among endome-trial thickness change groups (compaction, expansion, and stable groups) were 48%, 46%, and 28% respective-ly. Clinical pregnancy rates among endometrial volume change groups (decrease, increase and stable groups) were 82%, 2% and 18% respectively .In addition, logistic stepwise analysis for the effect of changes in endometrial indices on clinical pregnancy rate revealed that endome-trial volume change percent was the only significant vari-able predicting CPR (P<0.0001) and CPR was not sig-nificantly associated with endometrial thickness change (P=0.961) or endometrial blood flow change. ROC curve showed that endometrial volume change percent with cut off value of 10.44% had 72.55% sensitivity and 77.08% specificity for prediction of clinical pregnancy rate. 
Conclusion: The endometrial volume changes after progesterone administration was the only significant independent predic-tor of clinical pregnancy rate in FET cycles. Furthermore, a change in the endometrial volume of 10.44% was associated with sig-nificant improvement in clinical pregnancy rates of FET cycles with artificial endometri-al preparation.