Comparable live births for single vitrified blastocyst transfer from Day 5 versus Day 6: a retrospective analysis

Abstract Research question : Do blastocysts vitrified on Day 6 (D6) exhibit different clinical outcomes compared to blastocysts vitrified on Day 5 (D5) in vitrified-warmed single embryo transfers (SETs) without preimplantation genetic testing (PGT)? Design : We analysed 631 SETs involving vitrified D5 (n=541) or D6 (n=90) blastocysts. The primary outcome was live birth rate (LBR), and secondary outcomes included rates of biochemical pregnancy, clinical pregnancy and miscarriage, and neonatal birth weight. Multivariable logistic regression was used to adjust for clinically relevant patient-, treatment-, and embryo-related covariates selected a priori. Results : The LBR was 10.3% higher for D5 versus D6 blastocysts (30.3% versus 20.0%, p=0.045). The rates of biochemical pregnancy, clinical pregnancy and miscarriage did not significantly differ between the groups. Mean birth weight was comparable following transfer of D6 and D5 blastocysts (3640 g vs. 3440 g). Blastocyst vitrification on D6 did not significantly impact the odds of live birth in multivariable logistic regression analysis (OR 0.63, 95% CI 0.35–1.14). Multivariable logistic regression additionally revealed that live birth was less likely in women with endometriosis (OR 0.46, 95% CI 0.24–0.90, p=0.02) and women who had other reasons for infertility than secondary, male factor or unexplained infertility (OR 0.51, 95% CI 0.29–0.89, p=0.02), but was more likely in those receiving a short agonist protocol for stimulation (OR 1.63, 95% CI 1.02–2.61, p=0.04) and those with greater blastocyst expansion post-warming (OR 1.51, 95% CI 1.08–2.10, p=0.01). Conclusions : D6 blastocysts can be safely transferred with comparable clinical outcomes as D5 blastocysts. However, further studies are needed, and the importance of other factors in embryo selection should be emphasized.