Influence of post-thaw culture duration on pregnancy outcomes in frozen blastocyst transfer cycles

Background: To evaluate whether post-thaw culture duration affects clinical outcomes after frozen blastocyst transfer. Methods: : This retrospective cohort study included 3901 frozen-thawed blastocyst transfer cycles between September 2019 and March 2021. The cohorts were divided into two groups: developmental stage [day (D) 5 and D6] and culture duration after thawing (short culture of 2 to 6 h and long culture of 18 to 20 h). Women in the short culture group following D6 blastocyst transfer were further assigned into three subgroups depending on the post-thaw culture period (2 h, 4 h, and 6 h). The main outcomes were the live birth rate (LBR), implantation rate (IR), clinical pregnancy rate (CPR), and abortion rate (AR). Propensity score matching (PSM) analysis was applied to ensure comparability within groups. Multivariable regression analysis was used to study the effect of post-thaw culture duration on pregnancy outcomes. Results: : The IR, CPR, AR, and LBR showed no statistical differences within the groups following D5 blastocyst transfer. Compared with the short culture group, patients in the long culture group had significantly lower IR (35.5 vs. 45.8%, P < 0.001), CPR (45.3 vs. 56.6%, P = 0.001), and LBR (35.5 vs. 48.5%, P < 0.001), but a significantly higher AR (21.6 vs. 14.3%, P = 0.049) following D6 blastocyst transfer. However, data failed to present the superiority of any short culture duration over another on the live birth outcome for embryos vitrified on D6 (aOR 0.96, 95% CI 0.53-1.73, P = 0.881 for 4-h vs. 2-h subgroup; aOR 1.01, 95% CI 0.68-1.49, P = 0.974 for 6-h vs. 2-h subgroup). Conclusions: : Both post-thaw protocols can be applied to patients with D5 blastocysts. To optimize the pregnancy outcome following D6 blastocyst transfer, the short culture duration is recommended. All three short culture durations (2 h, 4 h, and 6 h) can be applied, and the choice is based on each laboratory’s workflow.