Synchronization between embryo development and endometrium is a contributing factor for rescue ICSI outcome.

Recent evidence shows that the outcome of rescue intracytoplasmic sperm injection (ICSI) is unsatisfactory on account of a poor clinical pregnancy rate. These outcomes may be due to either the in-vitro ageing of cultured oocytes before ICSI or the asynchrony between the embryo developmental stage and the endometrial secretory pattern. To address the latter issue, this study performed a retrospective analysis of 534 fresh cycles after rescue ICSI and 64 frozen–thawed cycles in subsequent treatment. Rescue ICSI cycles were divided into three groups: group I included 469 fresh embryo-transfer (FET) cycles; group II included 74 FET cycles in which supernumerary good-quality embryos were also cryopreserved; and group III included 64 frozen–thawed transfer cycles. Group III was considered to have achieved better synchronization than group II. As a result, significantly higher clinical pregnancy (29.69%, 19/64 versus 10.81%, 8/74) and implantation (13.33%, 22/165 versus 5.13%, 8/156) rates were achieved in group III compared with group II (both P<0.05). Therefore, synchronization of embryo development with the endometrium is considered a contributing factor for rescue ICSI outcome. It is recommended that embryos derived from rescue ICSI cycles should be cryopreserved and subsequently used in frozen–thawed cycles.