P-263 Assisted hatching, embryo quality, and re-expansion: Exploring their impact on frozen embryo transfer outcomes and predictive factors for implantation
Authors: García Esteve A , Escribá Suárez M , Alavés Navarro M , González Abreu D , Lozano Zamora M , López Martínez V , Aibar Manero J , Miret Lucio C , Benavent Martínez M , Crespo Simó J , Teruel López J 
Published: July 2024

Abstract

Study question
Do post-warming assisted hatching, embryo re-expansion grade and quality affect clinical pregnancy rates in frozen embryo transfer cycles?

Summary answer
Assisted hatching (AH) improves clinical outcomes in poor-quality embryos that struggle to hatch on their own. Embryo re-expansion grade and quality are predictive for implantation.

What is known already
In recent years, the use of frozen embryo transfer has grown considerably. Pre-vitrification morphological classification is a well-established method for assessing embryo quality and predicting treatment outcomes. However, there is limited evidence regarding the prognostic value of blastocyst characteristics after thawing.
Embryo vitrification has revolutionized assisted reproductive technology. However, this technique can stiffen the embryo’s zona pellucida, making hatching and implantation in the uterus more challenging. To facilitate these processes, AH is performed in laboratories. While research has been conducted on AH’s efficacy, controversy remains, and it is not yet clear when it is beneficial for embryos.

Study design, size, duration
Retrospective observational study of 486 patients undergoing single embryo cryotransfers between January 2022 and December 2023.
Patients were divided into two groups: AH-NO (n = 246) and AH-YES (n = 240). In the AH-YES group, a 1/4 opening of the zona pellucida was performed using laser shots (Octax-NaviLase®) after thawing.
Images of embryos cultured for at least 4 hours (x = 5.23 hours) in a time-lapse incubator (EmbryoScope+™, Vitrolife) were analysed from the time of thawing until the time of transfer.

Participants/materials, setting, methods
Blastocyst morphology was assessed on two categories: good quality (GQ) embryos (expansion 4-6, inner cell mass (ICM) and trophectoderm (TE) A/B) and poor quality (BQ) embryos (expansion 3-6, ICM and TE C) according to the Gardner-Schoolcraft classification.
Blastocyst re-expansion was evaluated in three categories: fully re-expanded (F), partially re-expanded, 30-70% re-expansion (P) and unre-expanded/collapsed (N) embryos following thawing before transfer.
Statistical analysis was performed using the Chi-squared test.

Main results and the role of chance
In the comparative analysis of clinical data regarding whether or not AH was performed after thawing, there were no significant differences in implantation rates (AH-YES: 58.33%, AH-NO: 58.54%, p &gt; 0.05) or miscarriage rates (14.39% vs 15.97%, p &gt; 0.05) between the two groups.
When comparing by embryo quality, there were no differences in implantation rates of good-quality embryos AH-YES: 60.59%(n = 216) vs AH-NO: 60.34%(n = 237). However, in poor-quality embryos, there was an improvement in the implantation rate in those in which AH had been performed, AH-YES: 41.67%(n = 24) vs AH-NO: 11,11%(n = 9), although no statistical difference was observed, likely due to the sample size.
When comparing by degree of re-expansion, there were significant differences (p &lt; 0.05) in implantation rates of completely re-expanded embryos AH-YES: 61.31% (n = 168) vs AH-NO: 62.93% (n = 205), partially re-expanded embryos AH-YES: 56.26% (n = 54) vs AH-NO 39.29% (n = 28), and unre-expanded or collapsed embryos AH-YES: 23.53% (n = 17) vs AH-NO: 33.33% (n = 12).

Limitations, reasons for caution
The study is limited by its retrospective nature and the low number of poor-quality embryos transferred as they are the last option for transfer. Additionally, it is common to transfer more than one poor-quality embryo to increase the chances of pregnancy, therefore losing implantation track.

Wider implications of the findings
Assisted hatching (AH) could benefit poor-quality embryos, and the degree of re-expansion before embryo transfer could be a predictor of implantation.

Trial registration number
not applicable