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? The outcomes depend on the degree of embryo development on Day 5. 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. This was a retrospective study that included 477 women aged 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 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. 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. No external funding was used for this study. The authors have no conflicts of interest. N/A.