Pregnancy outcomes of four different cycle protocols for frozen embryo transfer: a large retrospective cohort study

Objective: To evaluate the pregnancy outcomes of the four endometrial preparation protocols for people undergoing frozen-thawed embryo transfer (FET), including natural cycle (NC), hormone replacement therapy cycle (HRT), gonadotropin-releasing hormone agonist artificial cycle (GAC), and ovarian stimulation cycle (OC). Methods: This retrospective cohort study enrolled 10,333 cycles of frozen embryo transfer performed at Xinan Gynecological Hospital in Sichuan, China, from January 2018 to December 2018. The patient’s baseline characteristics and pregnancy outcomes were extracted from the medical record system. Pregnancy outcomes were compared among the four groups and multiple logistic regression models were used to adjust for the confounding factors. Results: After adjusting for covariates, multiple logistic regression analysis showed no statistical significance in pregnancy outcomes in the HRT group, GAC group, and OC group compared to the NC group in the entire population. The adjusted odds ratio of live birth was 0.976 (95% confidence interval [CI] 0.837–1.138) for the HRT group, 0.959 (95% confidence interval 0.797–1.152) for the GAC group, and 0.909 (95% confidence interval 0.763–1.083) for the OC group. Conclusions: The natural protocol had comparable pregnancy outcomes compared to the other three endometrial preparation protocols in the overall FET population. More high-quality prospective randomized controlled trials are required to assess the efficacy of the four protocols and explore the optimal one.