P-303 Outcomes of day 5 versus day 6 blastocysts: correlation of euploidy, implantation and embryo quality
Authors: Escriba M , Benavent M , Miret C , Garcia A , Gonzalez D , Crespo J , Teruel J 
Published: June 2023

Study question

Are outcomes of day 5 (D5) blastocysts better than day 6 (D6) blastocysts in a PGT-A program?

Summary answer

Outcomes of day 5 embryos show more blastocysts, better quality, more euploidy due to better quality and higher implantation on same quality than day 6.

What is known already

Human embryos optimally reach the blastocyst stage after five days of culture, but some have a slower development. The superiority of day 5 blastocysts compared to day 6 blastocysts in fresh cycle transfers was previously demonstrated and attributed mainly to endometrial asynchrony.

Data from frozen blastocysts transfers showed conflicting results, some studies have announced higher pregnancy rates after day 5 transfer compared with day 6 transfer, while others have shown equivalent outcomes.

However, none of these studies have compared outcomes between same quality and euploid day 5 (D5) frozen blastocysts versus those frozen on day 6 (D6).

Study design, size, duration

Retrospective observational study performed in a private centre between May 2017 and December 2022.

The study includes the data analysis of 5599 D5 and D6 blastocysts undergoing PGT-A obtained from 819 patients following 1295 PGS cycles. 948 euploid blastocysts with known implantation outcome were transferred in 789 frozen embryo transfers.

Blastocyst morphology was scored in 3 groups: A: excellent (AA, AB, BA), B: good (BB), C: average and poor-quality embryos (BC, CB, CC). (Gardner-Schoolcraft classification)

Participants/materials, setting, methods

PGT-A with NGS technology was offered to patients of advanced maternal age and/or with repeated IVF failures. Trophectoderm biopsies were performed on D5 and/or D6 embryos, with laser assistance. (Navilase, OCTAX)

We compared both populations (D5 and D6) in terms of number of blastocysts achieved and distribution of embryo quality of each population. As primary outcome, same-quality group results of euploid rate and implantation were compared according to the day of blastocyst development (D5 and D6).

Main results and the role of chance

Vast majority of the blastocysts were biopsied on D5 (71.8%, n = 4017) versus (28.2%, n = 1582) biopsied on D6.

The proportion of three quality-categories of blastocysts according to D5/D6 is uneven. Quality A = 18.6% vs 3,5%. Quality B = 72,4% vs 59,6%. Quality C = 9,0% vs 36,9%, respectively, showing significative differences between D5/D6 (Chi^2 p-values of 3 groups&lt; 0.001), concluding that overall D5 quality is better than D6.

Overall euploidy rate on D5 population versus D6 was 31,3% vs 23,3% (p value &lt;0.001), showing significant difference in favour of D5. But layering on 3 quality categories on D5/D6, the euploidy rate was 44,7% vs 51,8% for quality A, 29,6% vs 26,6% for quality B and 17,6% vs 14,9% for quality C, respectively, showing no significative differences D5/D6 (Chi^2 p-values: A = 0,331; B = 0,067; C = 0,280). Euploidy is not D5/D6 dependant but quality dependant.

Overall implantation on D5 (65,9%) is higher than D6 (40,4%) (p value &lt;0.001). Layering again, on D5 the implantation was 75,0% vs 66,7% for quality A, 63,8% vs 46,4% for quality B and 35,3% vs 26,3% for quality C, respectively. Despite implantation being higher on D5 on all three groups only group B shows D5/D6 significative differences. (Chi^2 p-values: A = 0,651; B &lt; 0.001; C = 0,360). Implantation is embryo quality and D5/D6 dependant.

Limitations, reasons for caution

The study is limited by its retrospective nature and the low number of grade A D6-euploid blastocysts available to transfer. Additionally, it is common to transfer more than one grade C quality embryo to increase the chances of pregnancy, losing implantation track of these type.

Wider implications of the findings

It was uncontested that D5 blastocysts had better reproductive potential, but we managed to quantify this potential according to euploidy and implantation rates based on embryo quality, which remains the most important predictive biomarker for selecting the best embryo to transfer and reducing the time to achieve a pregnancy.

Trial registration number

not applicable