P-143 Comparison of clinical outcomes between overnight warmed and embryo transfer day warmed blastocysts in frozen embryo transfer cycles
Authors: Chong Y  S , Chan M  L H , Liu S 
Published: July 2024

Study question

Is there any difference between clinical outcomes of overnight warmed (ONW) and embryo transfer day warmed (ETDW) blastocysts in frozen embryo transfer (FET) cycles?

Summary answer

ETDW blastocysts had better clinical outcomes compared to ONW blastocysts in FET cycles.

What is known already

Some studies reported that the resumption of mitosis in thawed cleavage-stage embryos resulted in significantly better clinical outcomes in FET cycles. Unnecessary transfer of non-functioning embryos can be avoided by incubating thawed embryos overnight to deselect embryos that developed abnormally or failed to undergo additional cleavage.

The synchronisation between a viable embryo and a receptive endometrium is vital for successful implantation to take place. The optimal implantation rates occur with embryo-endometrial developmental asynchrony of ± 1.5 days or less. The implantation rates are progressively reduced when embryo-endometrial development asynchrony is greater than 3 days.

Study design, size, duration

This was a retrospective study involving 2368 FET cycles in KKIVF Centre, KK Women’s and Children’s Hospital, Singapore from Jan 2021 to Dec 2022. The transfer of embryos or blastocysts derived from thawed oocytes, cleavage-stage embryos, double blastocysts and cultured blastocysts from thawed cleavage-stage embryos were excluded. Only single vitrified-warmed blastocyst transfers (n = 1445) were included in this study.

Participants/materials, setting, methods

Cryopreserved blastocysts were routinely warmed one day before embryo transfer (ET) using Vit Kit - Warm (FUJIFILM Irvine Scientific®) and incubated overnight (ONW group). However, when ETs fell on Monday or post public holiday, vitrified blastocysts were warmed on ET day (ETDW group). Regardless of blastocyst age, surviving blastocysts were transferred into day 5 endometrium. Statistical analysis was performed using t-test or Chi-square test. Differences with P &lt; 0.05 were considered as significant.

Main results and the role of chance

No statistical significant difference was observed in terms of maternal age distribution at the time of oocyte retrieval between ONW and ETDW groups (34.26 ± 3.29 vs. 34.49 ± 3.22, P = 0.221). Overall, clinical pregnancy rates (CPR) and live birth rates (LBR) were significantly higher in the ETDW group (CPR: 47.47% vs. 40.47%, P &lt; 0.01; LBR: 34.99% vs. 28.78%, P &lt; 0.05). Clinical outcomes were comparable in both ONW and ETDW groups for vitrified-warmed day 5 blastocyst transfers (CPR: 51.27% vs. 58.44%, P = 0.11; LBR: 37.06% vs. 44.81%, P = 0.07). CPR (45.02% vs. 25.71%, P &lt; 0.0001) and LBR (32.03% vs. 17.45%, P &lt; 0.0001) were significantly increased when vitrified day 6 blastocysts were warmed and transferred on the actual day of ET. 18.42% CPR and 13.16% LBR were achieved when vitrified-warmed day 7 blastocysts were transferred on the ET day itself. However, transfer of vitrified-warmed day 7 blastocysts in the ONW group did not result in any pregnancy.

Limitations, reasons for caution

Study subjects were not stratified according to patient’s age band and blastocyst quality. Age itself is an independent determinant of blastocyst quality which can affect clinical outcomes.

Wider implications of the findings

Warming of a vitrified blastocyst on ET day is preferred regardless of blastocyst age. However, to facilitate seamless operation in laboratory, overnight warming for vitrified day 5 blastocysts is feasible without compromising clinical outcomes.

Trial registration number

Not applicable