Impact of post‐warming culture duration and blastocyst morphology on reproductive outcomes in vitrified–warmed cycles: A retrospective cohort study

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AIM: The use of vitrified-warmed blastocyst transfer (VBT) cycles has increased significantly. This study aimed to investigate the effects of the post-warming culture duration and blastocyst morphology on clinical outcomes in VBT cycles. METHODS: We conducted a retrospective cohort study of 1551 VBT cycles. After propensity score matching to adjust for confounding variables, cycles were categorized based on post-warming culture duration into short-period (SPC; n = 365) and long-period (LPC; n = 730) culture groups. Blastocysts were classified into good (GG; n = 413) and poor morphological grade (PG; n = 289) groups. Clinical pregnancy and miscarriage rates were compared between groups to assess correlations with culture duration and morphological quality. RESULTS: In the GG group, post-warming culture duration was not significantly associated with survival, clinical pregnancy, implantation, ongoing pregnancy, or miscarriage rates for both single and double embryo transfers. However, in the PG group, the miscarriage rate was significantly higher in the SPC group than in the LPC group (26.7% vs. 2.6%). Furthermore, the SPC-PG group showed increased vulnerability in embryonic markers (positive markers: 82.5% vs. 69.5%; negative markers: 17.5% vs. 30.5%). CONCLUSIONS: Overall, no statistically significant differences were observed in clinical and ongoing pregnancy rates between the SPC and LPC groups. These findings suggest that while the timing of embryo transfer (ET) after warming can be adjusted to optimize each laboratory's workflow, the primary determinant of ET strategy should be blastocyst morphology, with culture duration tailored accordingly.