Comparison between two protocols for thawed embryo transfer: Natural cycle versus exogenous hormone replacement

Introduction: There are two most popular protocols for Frozen Embryo Transfer: the natural and the E2&P4 replacement cycles. There is still a controversy whether one is superior over the other. Purpose: To compare the outcome in patient groups undergoing FET following these protocols. Methods: About 1235 FET cycles were retrospectively analyzed during a period of 12 years. In 798 cycles (group A), the natural cycle protocol was used, and in 437 cycles (group B), the exogenous E2&P4 administration protocol was used. Results: The average patient age was 32.11±0.27 years in group A and 32.94±0.19 years in group B (p<0.05). The endometrial thickness was 9.54±0.11mm and 8.95±0.13mm in groups A and B, respectively (p<0.001). The peak serum E2 level was 162.51±8.97pg/mL and 250.78±33.67pg/mL in groups A and B, respectively (p<0.001). The implantation, clinical pregnancy, and ongoing pregnancy rates in groups A and B were 6.47%, 12.91%, and 10.4% versus 4.26%, 8.47%, and 5.95%, respectively (p<0.05). Conclusions: Natural endometrial preparation yields better outcome in compare with exogenous E2&P4 in FET cycles with higher endometrial thickness, implantation, and clinical pregnancy rates.