Effect of the Timing of Frozen Embryo Transfer on Pregnancy Outcomes in High Ovarian Response Patients Undergoing Freeze-All Strategy

Abstract Background: Frozen embryo transfer (FET) can greatly improve the pregnancy outcomes for high ovarian response (HOR) population. However, it is not known whether the impaired endometrial receptivity derived from controlled ovarian hyperstimulation (COH) can be fully recovered in the first menstrual cycle after oocyte retrieval, and whether the timing of FET is a risk factor on pregnancy outcomes in HOR population undergoing freeze-all strategy.Methods: A retrospective cohort study to compare the pregnancy outcomes of the immediate and delayed FET groups in HOR population undergoing freeze-all strategy. Propensity score matching was used to make the potential risk factors of the immediate and delayed FET groups comparable. Multivariable regression analysis was used to study the effect of the timing of FET on pregnancy outcomes in the entire cohort and propensity score-matched cohort, even in different COH protocol cohorts as subgroup analysis.Results: We showed that the immediate FET group were no worse than delayed FET group in the entire cohort [clinical pregnancy rate (CPR), adjusted odd ratio (OR), 0.942, 95% confidence interval (CI), 0.784-1.133; spontaneous abortion rate (SAR), adjusted OR, 1.118, 95% Cl (0.771-1.623); live birth rate (LBR), adjusted OR, 1.060, 95% Cl (0.886-1.267)]. The same results were obtained by χ2 test in the propensity score-matched cohort (CPR, 60.5% versus 63.5%; SAR, 11.6% versus 12.3%; LBR, 48% versus 49.3%) (P > 0.05). Subgroup analysis indicated that pregnancy outcomes of immediate FET were non-inferior to delayed FET in short-acting gonadotropin-releasing hormone agonist (GnRH-a) long protocol (P > 0.05). The SAR of the immediate FET group were lower than that of the delayed FET group in GnRH antagonist protocol (adjusted OR, 0.646, 95% CI, 0.432-0.966) and long-acting GnRH-a long protocol (adjusted OR, 0.375, 95% CI, 0.142-0.990) (P < 0.05), no differences were observed in CPR and LBR (P > 0.05).Conclusions: These findings indicate that immediate FET might not affect pregnancy outcomes in HOR patients undergoing freeze-all strategy. Delaying FET could increase the SAR in GnRH-ant and long-acting GnRH-a long protocols.