['To identify blastocyst features independently predictive of successful pregnancy and live births with vitrified-warmed blastocysts.', 'Retrospective study.', 'Academic hospital.', 'Women undergoing a cycle with transfer of blastocysts vitrified using the Rapid-i closed carrier (n = 358).', 'None.', 'Clinical pregnancy and live-birth rates analyzed using logistic regression analysis.', 'A total of 669 vitrified-warmed blastocysts were assessed. The survival rate was 95%. A mean of 1.7 ± 0.5 embryos were transferred. The clinical pregnancy, live-birth, and implantation rates were 55%, 46%, and 43%, respectively. The odds of clinical pregnancy (odds ratio [OR] 3.08; 95% confidence interval [CI], 1.88-5.12) and live birth (OR 2.93; 95% CI, 1.79-4.85) were three times higher with day-5 blastocysts versus slower-growing day-6 vitrified blastocysts, irrespective of patient age at cryopreservation. Blastocysts from multinucleated embryos were half as likely to result in a live birth (OR 0.46; 95% CI, 0.22-0.91). A four -fold increase in live birth was observed if an expanded blastocyst was available for transfer. The inner cell mass-trophectoderm score correlated to positive outcomes in the univariate analysis. The implantation rate was statistically significantly higher for day-5 versus day-6 vitrified blastocysts (50% vs. 29%, respectively).'] Are delayed blastulation , multinucleation , and expansion grade independently associated with live-birth rates in frozen blastocyst transfer cycles?