Delayed versus immediate frozen embryo transfer after oocyte retrieval: a systematic review and meta-analysis

Purpose This systematic review and meta-analysis aimed to compare pregnancy outcomes between immediate frozen embryo transfer (FET) performed within the first menstrual cycle after oocyte retrieval and delayed FET following subsequent cycles. Methods PubMed, EMBASE, and Web of Science were searched for eligible studies through January 2020. The main outcome measures were clinical pregnancy rate (CPR), live birth rate (LBR), and pregnancy loss rate (PLR). The effect size was estimated as risk ratio (RR) with 95% confidence interval (CI) using a random effects model. Inter-study heterogeneity was assessed by theI(2)statistic. Results Twelve retrospective cohort studies involving 18,230 cycles were included. The pooled results revealed no significant differences between delayed and immediate FET in CPR (RR 0.94, 95% CI 0.87-1.03;I-2 = 67.9%), LBR (RR 0.94, 95% CI 0.85-1.03;I-2 = 67.5%), and PLR (RR 1.05, 95% CI 0.87-1.26;I-2 = 42.7%). Subgroup analyses of freeze-all cycles showed a marginal decrease of CPR in delayed FET (RR 0.93, 95% CI 0.86-1.00;I-2 = 53.6%), but no significant changes were observed regarding LBR (RR 0.93, 95% CI 0.85-1.02;I-2 = 65.2%) and PLR (RR 1.09, 95% CI 0.84-1.41;I-2 = 59.1%). No statistical differences were found in effect estimates among other subgroup analyses by ovarian stimulation protocol, trigger agent, endometrial preparation regimen, and embryo stage. Conclusion Timing of the first FET after oocyte retrieval was not significantly associated with pregnancy outcomes. This finding refutes the current common practice to delay FET after oocyte retrieval and reassures patients who wish to proceed with FET at their earliest convenience. Due to the high heterogeneity and observational nature of included studies, further randomized controlled trials are needed to confirm the results.