Received March 01, 2026
Accepted March 09, 2026

Objective To investigate whether blastocyst diameter measured immediately prior to transfer and the occurrence of blastocyst collapse predict live birth outcomes after single blastocyst transfer (SBT). Methods This retrospective cohort study included 476 patients undergoing their first IVF/ICSI cycle with SBT on day 5. Blastocyst diameter was manually measured approximately 0.5 hours before transfer, and collapse events (≥50% reduction in blastocoel area) were assessed using time-lapse microscopy (TLM). Blastocysts were graded morphologically as Best, Good, or Fair. Multivariable logistic regression and Cochran–Armitage trend tests were used to evaluate associations with live birth. Results Live birth was achieved in 174 patients (36.6%). Blastocysts in the live birth group had significantly larger diameters (199.83 ± 18.68 vs. 192.34 ± 20.84 μm, P < 0.001) and a lower incidence of collapse (4.6% vs. 12.3%, P = 0.006). After adjusting for maternal age, endometrial thickness, BMI, FSH, and AMH, multivariable logistic regression showed that blastocysts with diameters >200 μm had a 2.49-fold higher likelihood of live birth compared to those ≤180 μm (95% CI: 1.46–4.25, P = 0.001), while collapsed blastocysts had 57% reduced odds (OR 0.43, 95% CI: 0.19–0.98, P = 0.044). Endometrial thickness (OR 7.31, 95% CI: 2.48–21.52, P < 0.001) and younger age (OR 0.94 per year, 95% CI: 0.89–0.99, P = 0.017) were also independently associated with live birth. A diameter-dependent increase in live birth rates was observed across all morphological grades (P < 0.05 for trend). Conclusion After adjusting for maternal age and endometrial thickness, blastocyst diameter measured near the time of transfer and the absence of collapse are independently associated with live birth after SBT, suggesting their potential as adjunctive selection criteria. These quantitative and dynamic parameters provide valuable adjuncts to conventional morphological grading, potentially improving embryo selection and IVF outcomes.