Received May 01, 2021
Accepted August 08, 2022

This study was aimed to determine whether developmental potential impacts the clinical outcomes, when good grade blastocysts of Day5 and Day6, were transferred in frozen embryo transfer (FET) cycles. A total of 654 women, including 460 (70.33%) Day5 and 194 (29.66%) Day6 were analysed, in which 905 Day5 and 274 Day6 ‎blastocysts were transferred. Only survived blastocysts of grade AA, AB, BA, BB quality and expansion grade between 3-6 (Gardner grading system) were included. The implantation rate was higher, 41.9% (379/905) in normal Day5 compared to delayed Day6 blastocysts transfers 36.5% (100/274), but not significant (p=0.1), the clinical pregnancy rate was similar and not significant (p=0.4) in normal Day5 (32.4%) compared to delayed Day6 (35%). Miscarriage rates were higher in normal Day5 (13.3%) compared to delayed Day6 (6.3%) blastocyst transfers but not significant (p=0.06). On the other hand, the biochemical pregnancy rate was significantly higher (p=0.001) in the delayed Day6 blastocysts (16.7%) transfer group compared to patients with normal Day5 (2.4%) blastocyst transfers. Two patients result in ectopic pregnancies from the delayed Day6 blastocyst transfer group. Live-Birth rates were significantly higher in Day5 blastocysts compared to Day6 (p=0.03). The developmental potential in embryos should not be considered a negative influence on pregnancy outcomes especially good grade blastocyst vitrified on Day5 and Day6. Fully expanded blastocysts on Day5 are considered similar in terms of outcomes to delayed Day6 blastocysts, however, live-birth rates are significantly higher in Day5 blastocysts.