Background: Frozen embryo transfer (FET) can greatly improve the pregnancy outcomes for high responder patients. However, it is not known whether the timing of FET is a risk factor on pregnancy outcomes in high responder patients undergoing freeze-all cycles.
Methods: A retrospective cohort study to compare the pregnancy outcomes of the immediate and delayed FET groups in high responder patients undergoing freeze-all cycles. The two groups were defined as that FET took place either within the first menstrual cycle following oocyte retrieval or afterwards. Propensity score matching was used to make the potential risk factors of the two 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 controlled ovarian hyperstimulation protocol cohorts as subgroup analysis.
Results: We obtained 1130 patients in immediate FET group and 998 patients in delayed FET group, and the average age of the two groups were 30.30 and 30.63. We showed that the immediate FET group were equivalent to delayed FET group in the entire cohort [clinical pregnancy rate (CPR), 61.0% versus 63.4%, adjusted odd ratio (OR), 0.939, 95% confidence interval (CI), 0.781-1.129; spontaneous abortion rate (SAR), 10.1% versus 12.6%, adjusted OR, 0.831, 95% Cl (0.628-1.098); live birth rate (LBR), 49.9% versus 49.2%, adjusted OR, 1.056, 95% Cl (0.883-1.263)]. 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 no difference to delayed FET in gonadotropin-releasing hormone agonist (GnRH-a) 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, 0.645, 95% CI, 0.430-0.966) (P < 0.05), no differences were observed in CPR and LBR (P > 0.05).
Conclusions: The pregnancy outcomes of immediate FET were no difference to delayed FET in high responder population undergoing freeze-all cycles.
Keywords: Frozen embryo transfer; GnRH analogue; High ovarian response; In vitro fertilization; Pregnancy outcomes; Timing.