Abstract
This study investigates whether there is an effect on laboratory results and clinical outcome using commercial kits with similar vitrification but different warming procedures for blastocysts vitrified on day 5 or day 6. A single-center retrospective cohort study was performed between 2011 and 2020. A change from a stage-specific kit (Kit 1) to a universal kit (Kit 2) was undertaken in 2017. A total of 1845 untested blastocysts were warmed for single vitrified-warmed blastocyst transfers (SVBT). Eight hundred and twenty-five blastocysts were vitrified with Kit 1 and 1020 with Kit 2. Blastocyst survival was not different (96.1% versus 97.3%). Seven hundred seventy-seven SVBT were performed from Kit 1 and 981 from Kit 2. Overall clinical pregnancy and live birth rates were not different (35.4% versus 34.1% and 30.9% versus 30.5% for Kit 1 and 2, respectively). Subgroup analysis for live birth rates in relation to the day of blastocyst vitrification showed no differences (36.1% and 36.1% for day 5 and 25.4% and 23.5% for day 6 blastocysts, respectively). For both kits, the mean gestational age was not different (38.8 ± 2.5 weeks versus 38.8 ± 2.0 weeks) with a singleton birth weight of 3413 ± 571 g and 3410 ± 528 g for Kit 1 and Kit 2, respectively. Differences in warming procedures do not affect laboratory performance or clinical outcome after blastocyst vitrification. The plasticity of a human blastocyst may allow for further investigation on simplification of blastocyst warming procedures.