FET-E2-raw data.doc

<b>Aim of the study</b> To evaluate the association between serum estradiol (E₂) and pregnancy outcomes of cleavage- or blastocyst-stage frozen embryo transfer (FET) cycles using hormone replacement therapy. <b>Methods</b> A total of 776 FET cycles (669 couples) performed from January 2016 to December 2019 were included in the present retrospective cohort study. The impact of progesterone-initiation-day serum E₂ levels on the ongoing pregnancy/live birth (OP/LB) rates was determined, and cleavage-stage embryo transfers and blastocyst-stage embryo transfers were analyzed separately. <b>Results</b> Regarding cleavage-stage embryo transfer cycles, serum E₂ levels on progesterone initiation day were significantly lower in the OP/LB group than in the non-OP/LB group (214.75 ± 173.47 vs. 253.20 ± 203.30 pg/ml; <i>P</i> = 0.023). In addition, there were downward trends in implantation, clinical pregnancy and OP/LB rates with increasing E₂ levels. However, in blastocyst-stage embryo transfer cycles, such trends were not observed, and there was no significant difference between the OP/LB group and the non-OP/LB group. Logistic regression analysis revealed that E2 levels on progesterone initiation day in cleavage-stage embryo transfer cycles were independently associated with OP/LB (odds ratio = 1.000, 95% confidence interval: 1.000-1.001, P = 0.008). The areas under the receiver operating characteristic curve were 0.55 in cleavage-stage embryo transfer cycles and 0.53 in blastocyst-stage embryo transfer cycles. <b>Conclusions</b> The results suggests that progesterone-initiation-day E₂ levels should be monitored during artificial cleavage-stage embryo transfer cycles. However, it is not necessary to monitor serum E₂ levels when transferring blastocysts in artificial FET cycles.