O-298 Embryo long-term storage does not affect ART outcome: analysis of 58,001 vitrified blastocysts
Authors: Bellver Pradas J , Coello A , De los Santos M  J , Meseguer M , Remohí J , Cobo A 
Published: July 2024

Abstract

Study question
Does duration of storage affect clinical outcome in frozen embryo transfer (FET) cycles?

Summary answer
Long-term storage does not affect live birth rate (LBR) in elective and non-elective FET cycles.

What is known already
Despite the success rates provided by the embryo vitrification programs, some studies found an association between the duration of cryo-storage and decreased clinical outcome, raising concern about the potential negative effect that prolonged storage may have on embryos.

Study design, size, duration
This was a retrospective multicenter study including a total of 58001 vitrified/warmed day-5 blastocysts from two different populations according to the reason for FET: elective FET which involved patients undergoing freeze all cycles (N = 16,615 blastocysts and 16,615 patients); and non-elective FET including 41,386 embryos from 25,571 patients. FETs were performed in the period between January 2010 and December 2020.

Participants/materials, setting, methods
Main outcome was live birth rate (LBR) and secondary outcomes were embryo survival, miscarriage, clinical and ongoing pregnancy rates. The impact of storage time and the potential confounding variables was assessed by univariate analyses. Those covariables found to significantly modify the LBR were included in a multivariable logistic regression. GEE (generalized estimated equation) was performed to allow the analysis of repeated measurements. Odds ratios (OR) and 95% confidence intervals (CI) were provided.

Main results and the role of chance
The blastocysts sample was clustered into deciles which provided specific storage duration categories that ranked from ≤0.67 to ≥ 4.34 and ≤1.8 to ≥ 34.81 months in freeze-all and non-elective FET, respectively. In the freeze-all group, maximum storage time was 34.8 months (2.9 years), while it was 121.2 months (10.1 years) in the non-elective FET group. Embryo survival did not show statistical differences across the categories of storage time in freeze-all and non-elective FET groups. In the freeze-all group, statistical differences were found for LBR across some, but not all the subgroups of storage duration. Similarly, in the non-elective FET group, LBR among the deciles showed statistical differences in some comparisons but not in all. However, after adjusting for confounding variables, the multivariate analysis showed no association between storage time and LBR in both groups. Blastocyst quality, body mass index (BMI), number of retrieved oocytes, endometrial preparation, male factor and uterine factor were related to the drop in the LBR in the freeze-all group (p &lt; 0.05). In the non-elective FET group, the variables that showed significant association with the LBR were age at retrieval and at FET, number of embryos transferred, number of retrieved oocytes, BMI, endometrial preparation, sperm origin and female factor.

Limitations, reasons for caution
The retrospective design of the study represents its main limitation.

Wider implications of the findings
Our results demonstrate the safety of long-term cryo-storage. This is reassuring for both IVF practitioners and patients especially considering the exponential increase in FET cycles observed in the last decades.

Trial registration number
Not applicable