Clinical Outcomes of Frozen-Thawed Single Blastocyst Transfers in Relation to Blastocyst Diameter and Timing of Cryopreservation

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Background: It is important to select blastocysts based on their likelihood of leading to pregnancy. The purpose of this study was to focus on expansion time and blastocyst diameter as simple factors to assess blastocysts relative to clinical outcomes. Methods: This study is based on 1,514 cycles in women [Formula: see text] years who underwent frozen-thawed blastocyst transfer (FBT) between November 2017 and February 2021. Blastocysts were classified according to the time they reached [Formula: see text]m inner diameter (116, 120, and 140[Formula: see text]h). Furthermore, blastocyst diameter at freezing was classified into three groups: groups S (under 167[Formula: see text][Formula: see text]m), M (167–181[Formula: see text][Formula: see text]m), and L (over 181[Formula: see text][Formula: see text]m), based on tertile values. We compared the clinical outcomes (including the clinical pregnancy rate [CPR]) of FBT. Results: CPR was significantly higher in groups M and L than in group S at 116[Formula: see text]h ([Formula: see text]). At 120[Formula: see text]h, CPR in group L was significantly higher than in group S ([Formula: see text]). However, at 140[Formula: see text]h, there were no significant differences among the three groups. There were no significant differences in miscarriage rates at 116, 120, and 140[Formula: see text]h. When only good quality embryos were compared, group L resulted in a significantly higher CPR than group S at 116[Formula: see text]h ([Formula: see text]). At 120[Formula: see text]h, both groups M and L had significantly higher CPR than group S ([Formula: see text]). There were no significant differences at 140[Formula: see text]h. Conclusion: These results suggest that when multiple embryos are frozen at the same time on day 5 of culture, a higher pregnancy rate can be obtained by selecting embryos with larger blastocyst diameters. In addition, it is important to confirm sufficient blastocyst expansion before cryopreservation.