O-160 Do day of vitrification, grade of expansion and blastocyst quality affect embryo survival rate? An observational study in oocyte recipient patients
Authors: Lopez Rodriguez M , Garijo López Y , Arnanz Poyatos A , Bronet Campos F 
Published: June 2023

Abstract

Study question
Which parameters are the most predictive of embryo survival rate in oocyte recipient cryotransfer cycles?

Summary answer
We have identified that only day of vitrification and grade of expansion have an impact on embryo survival rate

What is known already
Embryo vitrification is a well stablished technique that has led to survival rates around ±96%. Recent studies, based on day of vitrification, grade of expansion and blastocyst quality, are trying to identify which vitrified blastocyst has the highest potential to survive and to implant. However, controversial results are present in the literature, improvements in laboratory quality parameters and embryo culture could unleash into no significant differences regardless of morphological parameters. Nevertheless, having an acceptable survival rate does not mean to have a good implantation rate, biomarkers to identify the best potential blastocyst to survive and to implant are needed

Study design, size, duration
An unicenter-based retrospective observational clinical study was performed at IVI Madrid, approved by the local institutional review board (IRB). The study included all the warmings at blastocyst stage from oocyte recipient patients from January 2019 to April 2022. A total of 4.196 of warmed blastocysts were assessed, out of which 4.006 blastocysts were transferred

Participants/materials, setting, methods
All warmed blastocysts were allocated in groups according to the day of vitrification, grade of expansion and quality. The primary outcome analysed was embryo survival rate that was calculated as the number of viable embryos divided by the total number of embryos warmed. Vitrification and warming were performed following Kitazato method (Cryotop, Kitazato, Biopharma). Embryo classification was assessed previous vitrification according to IVIRMA criteria. Statistical analyses were conducted using univariate and multivariate regression model

Main results and the role of chance
The mean maternal oocyte recipient age was 42.27±3.89 years old and body mass index 23±3.74 kg/m2). In a multivariate logistic regression analysis, a statistically significant association between embryo survival rate and the day of vitrification and grade of expansion was found. Chance of survival was significantly reduced for blastocyst vitrified on day 6 (n = 737) compared to day 5 (n = 3459) (93,2 % versus 96,0% [OR = 0.976 (0.960-0.993); p=0.05]). Regarding grade of expansion, cavitated blastocyst (n = 840) survival rate was 97,7% [OR = 1;p&lt;0.001], expanded blastocyst (n = 1866) was 96,2% [OR = 0.095 (0.978-1.013)], hatching blastocysts (n = 1465) was 94% [OR = 0.978 (0.960-0.997); p=0.02) and fully hatched blastocysts (n = 25) was 60% [OR = 0.704 (0.648-0.765);p&lt;0.001)]. However, no association was found neither between lower inner cell mass quality [OR = 0.964 (0.842-1.104); p=0.596) nor trophoectoderm [OR = 1.004 (0.981-1.028); p=0.746)] and survival rate. Analysing the combination of these three variables will give us the possibility to choose the best embryo to warm. The impact of day of vitrification, grade of expansion and blastocyst quality on implantation, clinical pregnancy, miscarriage, and live birth rates will be included in further studies

Limitations, reasons for caution
A weakness of this study is that it is retrospective in nature. The single-group design employed in retrospective studies limits the researchers’ ability to determine cause and effect. Further studies are needed to determine the impact of these variables on the survival rate

Wider implications of the findings
Based on our findings, blastocyst survival rate is influenced mostly by the day of vitrification and degree of expansion. To be able to select the best blastocyst to warm, a comparison between different day of vitrification when we have a similar morphology and embryo quality cohort is needed

Trial registration number
Not applicable