P–254 The outcome of artificial reproductive technologies (ART) cycles with transfer of frozen-thawed blastocysts depending on whether expanded on day 5 or day 6
Authors: Polumiskova A , Tevkin S , Shishimorova M , Jussubaliyeva T 
Published: August 2021

Study question

Is there a difference in ART cycle results after frozen embryo transfer (FET), depending on whether blastocysts were cryopreserved on day 5 or 6?

Summary answer

There’s no statistical difference in the clinical pregnancy rate (CPR), life birth rate (LBR), miscarriage rate (MR) between embryos frozen on day 5 and 6.

What is known already

Currently, opinions differ regarding this topic. Previous studies demonstrated no difference in ongoing pregnancy rates between embryos frozen on day 5 (group A) or day 6 (group B) after FET. However, metanalysis (2019) suggested higher CPR and LBR after transferring embryos from group A rather than group B. It has also been established that ovarian stimulation leads to endometrial changes that result in deleterious effects on the implantation window and endometrial receptivity. Consequently, fresh transfers were excluded. Due to hormonal priming of endometrial receptivity, the same pregnancy outcomes should be expected with frozen-thawed blastocysts (day 5 vs 6).

Study design, size, duration

Retrospective cohort study was conducted between January 2015 and December 2018 with selected group of patients under 40 years of age. Group A consisted of 2275 cryotransfers of blastocyst expanded on day 5; group B included 170 cryotransfers of blastocyst on day 6. Both groups had an average of 1,52 embryos transferred per patient.

Participants/materials, setting, methods

Embryos were vitrified and warmed with Cryotop method (Kitazato, BioPharma). Blastocysts were scored according to Gardner and Schoolcraft grading system. Only expanded on day or 6 blastocysts of excellent and good (AA, AB, BA, BB) quality were selected. The embryos were cultured in CSC medium (Irvine Scientific) for 2–4 hours prior intrauterine transfer. The cycles with donor gametes, surrogacy and preimplantation genetic testing (PGT) were excluded. Statistical validity was assessed by Pearson’s chi-squared test.

Main results and the role of chance

The rates of the CPR, the ongoing pregnancy rate (OPR) and the LBR between group A and B were 50,8% (1157/2275) vs 46,5% (79/170) (p = 0,26), 37,4% (852/2275) vs 37,0% (63/170) (p = 0,91), 36,5% (832/2275) vs 35,2% (60/170) (p = 0,73) respectively and no significant differences were found in each category. Moreover, similarly there were no significant differences in the miscarriage rate 26,0% (301/1157) and 21,5% (17/79) (p = 0,37) as well

Limitations, reasons for caution

The study is limited due to uneven distribution of patients in both groups and by a low number of participants. The grading of blastocysts’ quality is also subjected to a human factor.

Wider implications of the findings: This study confirms that frozen-thawed blastocysts do not seem to exhibit a difference in the CPR, OPR, LBR and MR whether they were expanded on day 5 or day 6. The cryopreservation of day 6 blastocyst can increase the chances of the patient for the positive outcome.

Trial registration number

Not applicable