Comparison of natural and artificial cycles in frozen-thawed embryo transfer: A retrospective analysis of 1696 cycles.

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Objective:This study aimed to compare the pregnancy outcomes of natural cycles (NC) and artificial cycles (AC) in patients undergoing endometrial preparation for frozen-thawed embryo transfer (FET).Materials and Methods:This retrospective cohort study was conducted in a private infertility clinic between September 2016 and January 2021 and reviewed 1696 FET cycles. Among these FET cycles, endometrial preparation protocols that are performed as the NC (group 1) and AC (group 2) were analyzed. Outcome measures were live birth rates (LBR), clinical pregnancy rates (CPR), implantation rates (IR), and miscarriage rates (MR).Results:The mean serum estradiol level before progesterone supplementation was significantly higher in group 2, whereas endometrial thickness before progesterone supplementation was higher in group 1 (p<0.05). The mean number of transferred embryos and embryo quality score rates regarding cleavage and blastocyst stages were similar in both groups. The IR and MR were similar between groups (p>0.05). Additionally, CPR and LBR were similar in groups 1 (39.2% and 32.8%) and 2 (37.3% and 28.5%) (p=0.517, p=0.134, respectively). Multivariate logistic regression analyses revealed that female age at embryo freezing time and the number of transferred embryos were predictable variables of live birth [odds ratio (OR): 0.970, confidence interval (CI): 0.948-0.991, p<0.05, and OR: 1.359, CI: 1.038-1.780, p<0.05, respectively].Conclusion:Suitable endometrial preparation is essential to obtain successful pregnancy rates; however, no superiority was determined in NC or AC protocols in frozen-thawed cycles. One of these protocols may be performed depending on menstrual regularity and clinical experience.