BACKGROUND Controlled ovarian stimulation (COS) has a negative effect on the endometrial receptivity compared with natural menstrual cycle. Whether it's necessary to postpone the first frozen embryo transfer (FET) following a freeze-all strategy in order to avoid any residual effect on endometrial receptivity consequent to COS was inconclusive. OBJECTIVE The purpose of this retrospective study was to explore whether the delayed FET improve the live birth rate and neonatal outcomes stratified by COS protocols after a freeze-all strategy. METHODS A total of 4404 patients who underwent the first FET cycle were enrolled in this study between April 2014 to December 2017, and were divided into immediate (within the first menstrual cycle following withdrawal bleeding) or delayed FET (waiting for at least one menstrual cycle and the transferred embryos were cryopreserved for less than 6 months). Furthermore, each group was further divided into two subgroups according to COS protocols, and the pregnancy and neonatal outcomes were analyzed between the immediate and delayed FET following the same COS protocol. RESULTS When FET cycles following the same COS protocol, there was no significant difference regarding the rates of live birth, implantation, clinical pregnancy, multiple pregnancy, early miscarriage, premature birth and stillbirth between immediate and delayed FET groups. Similarly, no significant differences were found for the mean gestational age, the mean birth weight, and rates of low birth weight and very low birth weight between the immediate and delayed FET groups. The sex ratio (male/female) and the congenital anomalies rate also did not differ significantly between the two FET groups stratified by COS protocols. CONCLUSION Regardless of COS protocols, FET could be performed immediately after a freeze-all strategy for delaying FET failed to improve reproductive and neonatal outcomes.

中文翻译：

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冷冻胚胎的延迟转移未能改善需要全胚胎冷冻的患者的活产率和新生儿结局。

背景技术与自然月经周期相比，控制性卵巢刺激（COS）对子宫内膜容受性具有负面影响。是否有必要推迟采用全冻策略后的首次冷冻胚胎移植（FET），以避免因COS而对子宫内膜容受性产生任何残留影响。目的这项回顾性研究的目的是探讨采用全冷冻策略后，延迟FET是否可通过COS方案分层提高出生率和新生儿结局。方法在2014年4月至2017年12月期间，共有4404例患者经历了第一个FET周期研究，并分为即刻（停药后第一个月经周期内）或延迟FET（等待至少一个月经周期并将转移的胚胎冷冻保存少于6个月）。此外，根据COS方案，将每组进一步分为两个亚组，并按照相同的COS方案分析立即和延迟FET之间的妊娠和新生儿结局。结果当FET周期遵循相同的COS方案时，立即和延迟FET组之间的活产，植入，临床妊娠，多胎妊娠，早期流产，早产和死产的比率没有显着差异。同样，平均胎龄，平均出生体重，即时和延迟FET组之间的低出生体重和极低出生体重比率。在按COS方案分层的两个FET组之间，性别比（男性/女性）和先天畸形率也没有显着差异。结论无论采用哪种COS方案，都可以在延迟冻结FET的全部冻结策略未能改善生殖和新生儿结局后立即进行FET。