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Study Question: In IVF cycles in which the entire embryo cohort is slow growing, is it optimal to perform fresh transfer in Day 5 or to extend the culture and transfer in subsequent vitrified-warmed cycles?Summary Answer: The outcomes depend on the degree of embryo development on Day 5.What Is Known Already: Slow-growing blastocysts have lower implantation potential when transferred in fresh cycles. It has been suggested that embryo-endometrial asynchrony could explain this finding. However, studies that compared Days 5 and 6 embryos in frozen embryo transfer (FET) cycles showed contradictory results. There is still a lack of evidence regarding the best approach, performing fresh transfer or deferring transfer and continuing culture until fully developed blastocysts are achieved, when the entire cohort of embryos is slow growing.Study Design Size, Duration: This was a retrospective study that included 477 women aged <40 years who underwent fresh Day 5 single embryo transfer of slow-growing embryos and subsequent FET cycles of fully expanded blastocysts (FEB) originating from the same IVF cycle between 2012 and 2016.Participants/materials, Setting, Methods: The study included cycles in which the embryos either began blastulation by Day 5 of culture but did not reach the fully expanded stage (Gardner Stage III) or had delayed blastulation with only morula embryos present by Day 5 of culture. All of the subjects in the study underwent elective, single embryo transfer (slow or delayed blastocysts) on Day 5 and had at least one embryo that developed into a FEB on extended culture Day 6 that was suitable for vitrification. All subjects, regardless of the outcome of the fresh transfer, returned for at least one subsequent FET cycle of Day 6 embryos.Main Results and Role Of Chance: A total of 1070 embryo transfer cycles (fresh + FET) were included. Of them, 365 women had elective, fresh, single transfer of slow-growing blastocysts (Group I) and 112 had elective, fresh, single morula transfer (Group II). Groups I and II underwent a subsequent 457 and 136 FET cycles, respectively. The mean age of Group I was 33.8 ± 2.9 years, the proportion of Day 5 embryos that developed to FEB by Day 6 was 92%, and the number of blastocysts vitrified was 627 (average of 1.71 blastocysts per cycle). The outcomes of fresh and FET cycles were comparable regarding clinical pregnancy rate (CPR) (31.0 vs. 30.4%, P = 0.86) and live birth rate (LBR) (23.3 vs. 20.3%, P = 0.15). In Group II, the mean age was 35.8 ± 3.4 years and the proportion of morula embryos that developed to FEB by Day 6 was 72%. The number of blastocysts vitrified on Day 6 was 155 (1.38 per cycle). The transfer of fresh embryos in Group II resulted in significantly lower clinical pregnancy (5.3 vs. 30.1%, P < 0.001) and LBRs (1.8 vs. 20.5%, P < 0.001). The results did not change after controlling for possible confounding factors.Limitations and Reasons For Caution: The retrospective design of the study is a major limitation. Although we compared the outcomes of embryos that originated from the same cohort, the FET cycles could have been overrepresented by older patients and those with poorer prognoses. Furthermore, the study included only cycles in which there were blastocysts available for cryopreservation on Day 6; therefore, the results were not be applicable for those who had mandatory Day 5 transfer with no embryos available for vitrification.Wider Implications Of the Findings: Fresh transfer of embryos that begin blastulation on Day 5 results in similar outcomes to the transfer of FEB originating from the same cohort. However, in cases where only morula/compacting embryos are available by Day 5, extending culture until FEB are achieved and then performing subsequent FET will result in significantly higher LBRs.Study Funding/competing Interest(s): No external funding was used for this study. The authors have no conflicts of interest.Trial Registration Number: N/A. [ABSTRACT FROM AUTHOR]