Comparable live births for single vitrified blastocyst transfer from day 5 compared with day 6: a retrospective analysis

RESEARCH QUESTION: Do blastocysts vitrified on day 6 result in different clinical outcomes compared with blastocysts vitrified on day 5 in vitrified‒warmed single embryo transfers (SET) without preimplantation genetic testing? DESIGN: A total of 631 SET involving vitrified day 5 (n = 541) or day 6 (n = 90) blastocysts. Primary outcome was live birth rate (LBR); secondary outcomes were rates of biochemical pregnancy, clinical pregnancy, and miscarriage and neonatal birth weight. Multivariable logistic regression was used to adjust for clinically relevant covariates related to patients, treatments and embryos selected a priori. RESULTS: LBR was 10.3% higher for day-5 compared with day-6 blastocysts (30.3% versus 20.0%, P = 0.045). Rates of biochemical pregnancy, clinical pregnancy and miscarriage did not significantly differ between groups. Mean birth weight was comparable after transfer of day-6 and day-5 blastocysts (3640 g versus 3440 g). Blastocyst vitrification on day 6 did not significantly affect the odds of live birth in multivariable logistic regression analysis (OR 0.63, 95% CI 0.35 to 1.14). Multivariable logistic regression additionally revealed that live birth was less likely in women with endometriosis (OR 0.46, 95% CI 0.24 to 0.90, P = 0.02) and women who had other reasons for infertility than secondary, male factor or unexplained infertility (OR 0.51, 95% CI 0.29 to 0.89, P = 0.02), but was more likely in those receiving a short agonist protocol for stimulation (OR 1.63, 95% CI 1.02 to 2.61, P = 0.04) and those with greater blastocyst expansion after warming (OR 1.51, 95% CI 1.08 to 2.10, P = 0.01). CONCLUSIONS: Day-6 blastocysts can be safely transferred with comparable clinical outcomes as day-5 blastocysts. Further studies, however, are needed, and the importance of other factors in embryo selection should be emphasized.