P-140 Relationship between re-expansion rate of vitrified blastocysts during recovery culture post-warming and pregnancy rate after single blastocyst transfers
Authors: Morishita N , Miura M , Kobayashi Y , Matsunaga R , Maeda T , Ochi M , Horiuchi T 
Published: June 2023

Abstract

Study question
What is the relationship between re-expansion rate of vitrified-warmed blastocysts and pregnancy rate after single blastocyst transfers (SVBT)?

Summary answer
Re-expansion rate of vitrified-warmed blastocysts was higher with increasing the number of TEs and yielded higher pregnancy rate after SVBT.

What is known already
The increased use of blastocyst transfer has emphasized the importance of selecting high-quality embryos. Pre-vitrification morphological grading: Gardner’s criteria or time-lapse imaging of embryo morpho kinetics and developmental speed, is established a tool for selection of high-quality blastocysts. The recovery culture of vitrified blastocysts post-warming is performed before embryo transfer. Embryos that have dehydrated and shrunk during vitrification are restored to their pre-vitrification state during the recovery culture. Developmental competence in vivo of re-expanded blastocysts after thawing was higher than that of non-expanded blastocysts, but it is not clear the relationship between the re-expansion rate and the pregnancy rate, quantitatively.

Study design, size, duration
We analyzed 1034 vitrified-warmed blastocysts in 758 patients from July 2017 to June 2021. All embryos were monitored by a time-lapse system (EmbryoScope+, Vitrolife). Expanded blastocysts which reached the blastocoel cavity of 160 µm or more with obvious ICM were vitrified and cryopreserved. The blastocysts were transferred in the following cycle or later. Fetal heartbeat was detected as the pregnancy confirmation.

Participants/materials, setting, methods
The re-expansion rate was defined as the percentage of re-expanded blastocyst diameter after the recovery culture to the blastocyst diameter at the time of vitrification. The time of recovery culture was about 3 hours. The re-expansion rate and pregnancy rate were analyzed by logistic regression analysis. We also examined the association between re-expansion rate and the number of trophectoderm cells at equatorial region (eTE) or iDAScore: the parameters for selecting high-quality embryos.

Main results and the role of chance
Logistic regression analysis showed a significant relationship between re-expansion rate and pregnancy rate (P &lt; 0.0001), with an AUC of 0.58 and a cutoff value of 88.5%. Pregnancy rates in re-expansion rates ≥ 88.5% were significantly higher than those in &lt; 88.5% (51.7 vs. 37.4%). Therefore, we classified re-expansion rates ≥ 88.5% as positive and &lt; 88.5% as negative, and performed logistic regression analysis of women age, vitrification time of blastocysts, eTE, and iDASCore. The results showed that women age, vitrification time, and iDAScore all did not differ significantly from the re-expansion rate. The number of eTE was significantly higher with increasing re-expansion rates (P &lt; 0.0001), with an AUC of 0.58 and a cutoff value of 7.5.

Limitations, reasons for caution
We used only the expanded blastocysts with 160 µm or more in diameter and obvious ICM for vitrification. In addition, this study was retrospective, and further research is needed to confirm the reproducibility of the study in other clinics with different vitrification protocols and patient populations.

Wider implications of the findings
Quantitative analysis of re-expansion rate of vitrified blastocysts indicated that higher re-expansion rate (≥ 88.5%) had high pregnancy rate. It is useful to exploring the influence factors of re-expansion for producing high-quality embryos. Here, we found that the number of eTE is as an important factor for re-expansion.

Trial registration number
not applicable