P-191 Differences in morphokinetic patterns and clinical outcomes between fresh and frozen oocytes; a retrospective analysis

Abstract Study question Is there a difference in morphokinetics and clinical outcomes between embryos from fresh and vitrified oocytes? Summary answer Embryos from vitrified versus fresh oocytes showed a delay at the cellular stage, but no impact on time to blastulation or clinical outcomes was evident. What is known already Oocyte vitrification has greatly impacted assisted reproduction, with the number of treatments cycles using frozen oocytes more than doubling in the UK since 2013. Studies of thawed vitrified oocytes have shown similar success rates and outcomes compared to fresh, allowing the technique to be considered safe and effective. However, vitrification and thawing subjects the oocyte to stress and osmotic changes that may be evident in alterations in the timing of their morphological events. Analysis of morphokinetic markers using time-lapse incubators was performed to investigate this. Study design, size, duration Matched cohort study. A total of 823 embryos were analysed, 414 embryos from fresh oocytes and 409 from vitrified. The embryos were from the 288 ICSI treatment cycles performed at LWC in 2019. Fresh oocytes were from women less than 35 years old undergoing fertility treatment and vitrified oocytes were from egg donors under 35. Participants/materials, setting, methods Embryos graded AA, BB, BA, AB, were selected and annotated retrospectively on the Embryoscope for the following events: pronuclei appearance (tPNa) and disappearance (tPNf), time until two (t2), four (t4) and eight cells (t8), compaction initiation (tSC), the start of blastulation (tSB) and time to expanded blastocyst (tEB). PN duration, second and third embryo cell cycle (ECC), compaction and blastulation duration were also calculated as well as differences in clinical outcomes. Main results and the role of chance Embryos derived from vitrified oocytes (EVO) were observed to have a statistically significant delay in 4/8 morphokinetic events studied: t4 (p = 0.03), t8 (p < 0.01), tSC (p < 0.01) and tSB (p = 0.01). A mean delay of 1h50min was observed when compared to embryos from fresh oocytes (EFO). ECC duration showed a statistically significant difference with a delay of 48 minutes in the vitrified group. However, compaction occurred on average just 84min faster in this group, meaning no differences were observed in the time needed to achieve a full expanded blastocyst. Regression analysis revealed a correlation between the age of the oocyte and morphokinetic timings. Oocytes from older women demonstrated slower development, with age having a statistically significant impact in the following categories: tPNa, tPNf, t2 and t4. No differences found between fresh and vitrified groups in fertilization rate (80% EFO vs 79% EVO) (p = 0.841), embryo utilization rate (60% EFO and 61% EVO) (p = 0.432), implantation rate (54% EFO vs 52% EVO) (p = 0.837) and clinical pregnancy rates (49% EFO vs 42% EVO) (p = 0.502). Limitations, reasons for caution Limitations of the present study include the retrospective analysis, small sample size and the lack of adjustment for potential contributory/confounding factors such as semen quality, body mass index (BMI), antimüllerian hormone (AMH) levels, type of ovarian stimulation or type of infertility which are known possible influencers of embryo morphokinetics. Wider implications of the findings The delay observed at the cellular stage by EVO had no impact on the time the embryos needed to achieve full expansion. While vitrification affects embryo morphokinetics, it does not seem to impact the ability of the oocyte to be fertilized, activated, or to produce a viable blastocyst and pregnancy. Trial registration number Not applicable