P–264 Clinical relevance of re-expansion after blastocyst thawing
Authors: Aparici. González M , Herrer. Grassa L , Cascale. Romero L , Lláce. Aparicio J , Te. Morro J , Bernabe. Pérez R 
Published: August 2021

Abstract

Study question
Are there any differences in clinical outcomes after SET of re-expanded versus non-re-expanded blastocysts?

Summary answer
The transfer of re-expanded thawed blastocysts is associated with improved clinical outcomes.

What is known already
Improvements in embryo culture conditions, endometrial receptivity protocols and vitrification as a revolutionary cryopreservation technique have allowed the expansion of blastocyst stage transfers (Lieberman and Tucker, 2006; Stanger et al., 2012; Rienzi et al., 2017), increasing clinical pregnancy and implantation rates in IVF cycles.
The re-expansion of thawed blastocyst at the time of transfer has been considered as a good prognosis factor, but not always thawed embryos re-expand. To evaluate the relevance of this event, we compared the clinical results of the re-expanded embryos versus the collapsed ones after their thawing and transfer.

Study design, size, duration
A total number of 1.125 frozen-thawed blastocyst transfers were included in this retrospective observational study between January 2018 and December 2020. Seven hundred and eighty-six thawed blastocyst were fully expanded at the time of the transfer and 339 thawed blastocysts were non-re-expanded when they were transferred.

Participants/materials, setting, methods
1.125 single frozen-thawed blastocyst embryo transfer (SET) cycles (802 from donated and 319 from autologous oocytes) were divided in two groups (re-expanded vs non-re-expanded). Positive beta human chorionic gonadotrophin (bHCG), pregnancy rate (PR), early miscarriage rate (EMR) and live birth rate (LBR) were compared between the two groups.
Blastocysts were thawed using an Irvine Scientific® Thaw kit, Irvine Scientific® and were transferring in culture medium (Global® Total® LP, CooperSurgical®).

Main results and the role of chance
During 2018, 190 re-expanded blastocyst and 94 non-re-expanded were transferred. Statistical significant differences were found in the percentage of positive bHCG (48.4% vs 30.9%, p &lt; 0,0048) and PR (39.5% vs 25.5%, p &lt; 0,0203), respectively.
In 2019, statistical differences were found in the LBR between 307 re-expanded blastocyst and 124 non-re-expanded (30.6% vs 12.9%; p &lt; 0,00001). Differences were also found in positive bHCG (50.2% vs 21.8%, p &lt; 0,00001) and PR (40.7% vs 15.3%, p &lt; 0,00001), respectively.
Finally, in 2020, 289 re-expanded blastocyst and 121 non-re-expanded were transferred, and significant differences were obtained in the percentage of positive bHCG (46.8% vs 22.3%, p &lt; 0,00001) and PR (32.9% vs 15.7%, p &lt; 0,00001), respectively. Globally, all the variables analysed were statistically significant in favour of the re-expanded embryo group: positive bHCG (48.7% vs 24.5%; p &lt; 0,00001), PR (37.5% vs 18.3%; p &lt; 0,00001) and LBR (20.1% vs 9.5%; p &lt; 0,00001), except for EMR.

Limitations, reasons for caution
The inherent limitations to a retrospective design. Larger studies are warranted in order to reach robust conclusions on the subject.
Wider implications of the findings: Transfer of re-expand blastocyst could be a positive indicator of clinical outcomes. In case of non-re-expand embryos, transfer of two could be reasonable.

Trial registration number
NONE