Objective To assess cycle outcome among day 5 and day 6 cryopreserved frozen-thawed blastocyst embryo transfers (FBET). Design Retrospective cohort study. Setting Military-based ART center. Patients One hundred seventy-two non-donor, programmed cryopreserved embryo cycles. Interventions Fully expanded blastocysts on day 5 were cryopreserved on day 5 and those achieving this state on day 6 were cryopreserved on day 6. Leuprolide acetate was given for ovulation inhibition and endometrial supplementation was by oral and vaginal estradiol. Progesterone in oil was administered and blastocyst transfer occurred in the morning of the 6th day of progesterone. Main Outcome Measures Implantation, pregnancy and live birth rates. Results Fresh and frozen cycle characteristics were similar between groups. Day 5 FBET had significantly higher implantation rates (32.2% vs. 19.2%; p=0.01) which remained significant even when adjusting for co-variates (OR: 1.91; 95%CI: 1.00, 3.67). Live birth rates trended towards improvement after adjusting for co-variates (OR: 1.18; 95%CI: 0.61, 2.30). Conclusions Cryopreserved day 5 blastocysts have higher implantation rates and trend toward improved pregnancy outcomes compared to cryopreserved day 6 blastocysts. This suggests that embryo development rate may, in part, predict implantation and subsequent FBET outcomes although embryos not achieving the blastocyst stage until day 6 still demonstrate acceptable outcomes.