Abstract: Objective To explore the developmental potential of embryos at different developmental days and provide evidence
for blastocyst culture of non-top quality cleavage stage embryos in frozen-thawed embryo transfer (FET) cycles. Methods The
clinical data of 687 FET cycles were retrospectively analyzed. According to the embryo freezing time, the patients were divided
into day 5 (D5) blastocyst group (n=87), day 6 (D6) blastocyst group (n=111) and day 3 cleavage-stage embryo (D3) group (n=
489) with hormone replacement cycles or natural cycles for endometrial preparation. The clinical pregnancy rates, miscarriage
rates, and implantation rates were compared between the 3 groups. Results The clinical pregnancy rate, miscarriage rate and
implantation rate per transfer were 58.6%, 9.8%, and 42.9% in D5 group, 32.4%, 19.4%, and 23.3% in D6 group, and 44.9%,
16.4%, and 26.9% in D3 group, respectively. The clinical pregnancy rate and implantation rate were significantly higher in D5
group than in the other two groups (P<0.05). Conclusion The D5 blastocysts derived from non-top quality D3 embryos after
cryopreservation can have better clinical outcomes than those derived from D3 cleavage-stage embryos and D6 blastocysts,
and are therefore a better option than D3 cleavage-stage embryos in FET cycles.