P–226 Failure of blastocoele expansion within the first two hours post thawing could halve the chances of implantation

Study question

The aim of this study was to evaluate the influence of blastocoele re-expansion time of warmed vitrified blastocysts on clinical pregnancy outcome.

Summary answer

Clinical pregnancy rate was significantly higher after transfer of warmed vitrified blastocysts that were fully expanded within 2 hours post thaw.

What is known already

The number of blastocysts being vitrified worldwide has increased dramatically over recent years. A combination of factors has led to this including the introduction of vitrification, an increase in freeze-all policies, single embryo transfer and an increase in preimplantation genetic testing. Currently, blastocyst re-expansion after thawing is used to indicate the survival status of the blastocyst and when combined with the morphology of blastocyst can predict its reproductive potential. While time taken for blastocoele re-expansion has been proposed to be a biomarker of viability, its value in clinical practice remains unclear.

Study design, size, duration

This retrospective study analysed outcomes in patients who had frozen embryo transfers between June-December 2020. 233 embryos were reviewed with time-lapse to assess their blastocoele expansion post-warming and three groups were identified. The first included fully expanded blastocysts post-warming. The second group included partially expanded blastocysts and the third non-expanded blastocysts. In addition, the groups were subcategorised into two further categories depending on whether they took less or more than 2 hours to complete expansion.

Participants/materials, setting, methods

233 vitrified/warmed embryos from 216 patients were analysed using time-lapse incubators. The first group included 134 blastocysts, of which 70 were fully expanded within 2 hours and 64 after 2 hours post thaw. The second group had 70 embryos of which 45 expanded partially within 2 hours and 25 after 2 hours. The third had 28 embryos that had no expansion within the first 2 hours (n = 20) or after 2 hours (n = 8).

Main results and the role of chance

Blastocysts were collapsed by laser prior to vitrification. Single blastocyst transfer was performed for all patients. The mean transferred embryo age was 32.1± 5.5 and the recipient’s was 37.5± 5.9. Fully expanded blastocysts (n = 70) within 2 hours demonstrated a clinical pregnancy rate (CPR) of 57% compared with 38% from those that expanded fully after 2 hours (n = 64) (p = 0.02). Blastocysts with some form of expansion (full or partial) within 2 hours post-warming (n = 115) were associated a significantly higher CPR compared to those expanding after 2 hours (n = 89). The CPR was 55% and 39% respectively (p = 0.02). Embryos that showed no expansion (n = 20) within the first 2 hours post thaw resulted in CPR of 28%. Interestingly, embryos that showed no expansion after 2 hours resulted in no pregnancy. When combining morphology as a selection criterion, expansion within 2 hours of thawing was associated with a CPR of 62.5% for ≥4AB embryos, 50% for BB embryos and 45% for poorer embryos ≤CB.In conclusion, failure of blastocoele expansion post 2 hours reduced by half the chances of clinical pregnancy (p = 0.03). Combination of the degree of re-expansion and embryo morphology is an important predictor tool to improve clinical outcomes in frozen embryo transfers.

Limitations, reasons for caution

This study uses a small sample size of patients. The data are observational and were retrospectively analysed so unknown confounders could not be assessed. The addition of more cycles and further multivariate analysis, is essential for confirmation of the findings. However, initial results are very reassuring.

Wider implications of the findings: The degree of speed of re-expansion post warming should be used as a predictor for prioritisation of embryos for transfer. Owing to these preliminary findings there is rationale for a larger scale study combining other morphological indicators that could further assess implantation indicators and assist patient counselling

Trial registration number

Not applicable