Does oocyte vitrification affect morphokinetics of subsequent embryo development?

Study question Are the morphokinetic profiles, as assessed using time-lapse technology, of human embryos developed from vitrified oocytes different to those from fresh oocytes. Summary answer Vitrification of oocytes does have an effect on early developmental morphokinetic profiles, but this is normalized by the time the embryo has reached blastocyst. What is known already Vitrification of oocytes is now commonplace, but little is known about the effect this may have on subsequent embryo development. Study design, size, duration This was a retrospective data analysis, from 8 fertility clinics in the UK between 2012 and 2019. Embryos from patients in the vitrified group (n = 557) were matched to fresh patient controls (n = 539). The matching was performed based on the following criteria: type of treatment, patient age, cause of infertility and number of embryos. Participants/materials, setting, methods The embryos in each group were compared for mean morphokinetics of key developmental stages in hours post insemination (hpi). Parameters compared included early cleavage divisions (t2-t8), time to start of compaction (tSC), time to morula (tM), time to start blastulation (tSB), time to full blastocyst (tB) and duration of compaction (tB-tSC). Treatment outcomes were compared between the two groups, including percentage of blastocyst formation, clinical pregnancy rate, implantation rate and live birth rate. Main results and the role of chance The results showed a significant delay across all early cleavage divisions as follows for vitrified and fresh oocytes respectively: 2-cell (28.14 vs 26.10 (p < 0.001)), 3 cell (37.56 vs 35.37 (p < 0.001)), 4 cell (40.58 vs 37.54 (p < 0.001)), 5 cell (50.31 vs 47.14 (p < 0.001)), 6 cell (53.99 vs 50.87 (p < 0.001)), 7 cell (57.08 vs 54.48 (p < 0.001)) and 8 cell (61.26 vs 58.91 (p < 0.01)). In addition, tSC was also significantly delayed in the vitrified group (80.65 vs 76.36 (p < 0.001)). However, the compaction stage was significantly shorter in the vitrified oocytes (19.02 vs 22.45 (p < 0.001)). Therefore, there was no difference in the time that embryos derived from fresh and vitrified oocytes reached the blastocyst stage (108.03 vs 107.78 (p > 0.05)). No difference was found in clinical pregnancy, implantation or live birth rates but significantly fewer blastocyst developed from vitrified oocytes compared to fresh (36.09% vs 42.4% (p < 0.05)). Limitations, reasons for caution Although this was a matched analysis, it was a retrospective in nature therefore is subject to confounders. However, it would be problematic to perform a prospective randomized controlled trial to address this study question given the need to randomize patients to elective freezing of oocytes prior to embryo creation. Wider implications of the findings: Vitrification of oocytes may affect early developmental morphokinetic profiles, but any effect is normalized by the time the embryo has reached blastocyst. However, fewer blastocysts may develop following oocyte vitrification. This may have implications for oocyte donation banks and those patients choosing to cryopreserve oocytes. Trial registration number NA