P-129 Transferring blastocysts derived from frozen-thawed cleavage embryos versus frozen-thawed blastocysts: A comparative analysis of pregnancy outcomes
Authors: Tran H  P , Hoang T  T D , Tran H  L B , Trang H  N K , Do V  N T , Mai C  K , Dang Truong S 
Published: July 2024

Study question

Is there an enhancement in pregnancy outcomes when cleavage-stage embryos are thawed, cultured for transfer as blastocysts, as opposed to transferring thawed blastocysts?

Summary answer

The strategy of culturing frozen-thawed cleavage embryos for 2 days and transferring them as blastocysts is not inferior to the transfer of frozen-thawed blastocysts.

What is known already

Embryo cryopreservation in IVF has many advantages, allowing couples to postpone pregnancy for personal reasons and reducing the risk of ovarian hyperstimulation syndrome. In the past, the common practice involved freezing and transferring cleavage embryos. However, a current trend favors single-embryo transfers at the blastocyst stage, promoting potential self-selection during extended culture. Several studies have raised concerns about potential damage during blastocyst vitrification, prompting investigation into whether cryopreserving cleavage-stage embryos for subsequent culturing to blastocysts yields superior outcomes compared to direct transfer of thawed blastocysts.

Study design, size, duration

This observational study was conducted at Hung Vuong Hospital from January 2022 to December 2023. We included 509 patients who underwent IVF followed by FET cycles. The D3-5 group (n = 167) comprised embryos that were frozen and thawed at the cleavage stage, subsequently extended two more days of culturing to the blastocyst stage for transfer. The D5 group (n = 342) involved frozen and thawed blastocysts. We excluded donor cycles, surrogacy, and cycles with preimplantation genetics testing.

Participants/materials, setting, methods

We used vitrification method for embryo cryopreservation. The freezing and thawing process were performed using either Kitazato or Cryotec medium kit, following manufacture’s instruction. Regarding the D3-5 group, thawed cleavage embryos were cultured for two more days until blastulation and transfer. Regarding the D5 group, blastocysts transfer was performed at least 2 hours after warming. The embryo transfer procedure was performed under abdominal ultrasonographical guidance. Pregnancy results were recorded and compared between two groups.

Main results and the role of chance

Regarding the D3-5 group, a noteworthy proportion of 65.3% cycles were unfortunately cancelled. The primary reason for cycle cancellations was the absence of developed blastocysts for transfer (85.3%). The remaining 14.7% of cancellations were attributed to other reasons. Fertilization rates were comparable between the two groups, with 80.8% in the D3-5 group and 81.5% in the D5 group (p = 0.464). However, the blastulation rate in the D3-5 group (40.1%) was lower than in the D5 group (46.4%), (p = 0.016). Patients in the D5 group demonstrated slightly better pregnancy outcomes compared to the D3-5 group: implantation rate (59.9% versus 51.7%, p = 0.302), clinical pregnancy rate (52.3% versus 44.8%, p = 0.36), ongoing pregnancy rate (37.7% versus 34.5%, p = 0746), live birth rate (33.9% versus 31%, p = 0.78), and miscarriage rate (26.3% versus 20.7%, p = 0.46). However, these differences were not statistical significance.

Limitations, reasons for caution

This is an observational study, we observed that thawing the cleavage-stage embryos then subsequently culture to the blastocyst stage may lead to the lower blastulation rate or even absence of developed blastocysts for transfer. This approach also increases the workload for embryologists.

Wider implications of the findings

Culturing frozen-thawed cleavage embryos for 2 days and transferring them as blastocysts increases workload for embryologists and poses a risk of cycle cancellation. We propose that the use of frozen-thawed blastocysts may be a more efficient and patient-friendly option.

Trial registration number

not applicable