The impact of completely shrunken blastocysts on clinical outcomes in vitrified-warmed single blastocyst transfer cycles.

The aim of this study was to evaluate whether blastocysts that do not re-expand 2–4 h after thawing were of value for utilization and to analyze the relevant factors affecting clinical pregnancy in completely shrunken blastocyst transfer (CSBT) cycles. The retrospective cohort study included 104 single embryo transfer cycles with CSBT and 2172 cycles with re-expanded blastocyst transfer (REBT). Confounders between groups were adjusted using propensity scoring matching. Clinical pregnancy rate (CPR), ongoing pregnancy rate (OPR), and live birth rate (LBR) were the primary outcome measures. In addition, patients’ characteristics were compared between the pregnancy and non-pregnancy groups in CSBT cycles. After matching, CPR (28.8% vs. 61.5%), OPR (22.1% vs. 52.9%), and LBR (20.2% vs. 50.0%) were significantly lower in the CSBT group than in the REBT group,  < 0.001. In CSBT cycles, pregnancy and non-pregnancy groups had significant differences in maternal age (29.4 ± 4.5 vs. 32.4 ± 6.0,  = 0.007), basal FSH (6.8 ± 2.0 vs. 8.0 ± 3.7,  = 0.029), blastocysts derived from good-quality day 3 embryos (63.3% vs. 32.4%,  = 0.004), and blastocysts formed day 5 (80.0% vs. 50.0%,  = 0.005). Binary logistic regression analysis identified the day of blastocyst formation as a significant determinant of pregnancy outcomes. The clinical pregnancy rate was 3.062 times higher for day 5 blastocysts compared to day 6 blastocysts in CSBT cycles (adjusted OR 3.062, 95% CI 1.077–8.704,  = 0.036). In conclusion, blastocysts which did not re-expand within 2–4 h post-thawing should not be considered non-viable. Although clinical pregnancy and live birth rates are significantly lower in completely shrunken blastocysts (CSBs) compared to re-expanded blastocysts (REBs), CSBs still retain implantation potential, particularly when derived from day 5 embryos.