P-169 Low 5% VS ultra-low 3% O2 concentration on embryo culture: Is there any difference in quality and ploidy?
Authors: Papadopoulou M.I , Karagianni M , Vorniotaki A , Oraiopoulou C , Christophoridis N , Papatheodorou A , Chatziparasidou A 
Published: June 2022

Abstract

Study question
Can embryo culture in 3% O2 concentration improve fertilization and blastocyst formation rates, as well as euploidy rate, compared to 5% O2 embryo culture?

Summary answer
Ultra-low oxygen concentration (3%) does not seem to improve fertilization and euploidy rate neither embryo quality and blastocyst formation rate.

What is known already
Low oxygen tension (5%) during embryo culture has found to be beneficial for embryo development, when compared to atmospheric O2 tension (20%). Physiologically, oxygen tension is decreasing from ∼5% in the fallopian tube to ∼2,5% in the uterus and embryos enter the uterus on the post-compaction stage, around Day 4 of their development. Additionally, our previous study demonstrated that euploidy rate was significantly higher in 5% O2 concentration compared to 20%. Thus, the purpose of this study was to evaluate whether culture of embryos to blastocyst stage could benefit from a decrease in O2 concentration, from 5% to 3%.

Study design, size, duration
This retrospective study, included 176 PGT-A cycles, performed at Embryolab fertility clinic from May 2017 to November 2021. Two study groups were formed, in which oxygen tension was the only different parameter of culture conditions; the 5% O2 Group consisted of 114 PGT-A cycles resulting in 832 embryos and the 3% O2 Group with 62 PGT-A cycles including 479 embryos. Euploidy, fertilization and blastocyst formation rates were compared between the two groups.

Participants/materials, setting, methods
In all cycles included in the study, injected oocytes were incubated to either 3% O2 or 5% O2. The CO2 concentration was 6% in both groups. Embryos were cultured to blastocyst stage. In good quality available blastocysts (expansion: &gt;3, ICM: A/B, TE: A/B according to Gardner Grading System), trophectoderm biopsy was performed on Days 5/6 and embryos were analyzed for chromosomal abnormalities. All embryos were vitrified after biopsy. Statistical analysis was performed using Mann-Whitney test.

Main results and the role of chance
Mean female age was similar between 5% and 3% group (37.76±4.7y vs 38.35±4.19y, p = 0.2263).
Fertilization rates were similar between two groups (81.04% in 5% VS 81.77 in 3%, p = 0.9718). In 5% O2 Group, cleavage rate (98.86%) was found to be significantly higher, compared to 3% O2 Group (96.86%) (p = 0.003). Blastocyst formation rate (expressed per fertilized oocyte) was found to be significantly higher in the 5% O2 Group (76.56 in 5% VS 67.22 in 3%) (p = 0.029), as well as biopsied blastocyst rate (91.9% in 5% VS 82.29% in 3%) (p = 0.0056).
Euploidy rates were found to be similar between the two groups, 26.7 in 5% Group and 26.76 in 3% Group (p = 0.9272). However, more blastocysts were available for biopsy on Days 5/6 in the 5% Group, as shown above.
In the 5% Group, 101 embryotransfers have been performed, 54 with positive βHCG and 42 cycles had no euploid embryos after PGT-A. In the 3% Group, 28 embryotransfers have been performed, 18 with positive βHCG, 7 cycles are pending and 24 had no euploid embryos. This is an ongoing study and clinical outcomes remain to be reported and analyzed after the completion of data selection.

Limitations, reasons for caution
This is a retrospective study in which cycles from the two groups were not performed simultaneously but at different time periods. In addition, all PGT-A cycles were included in the study, regardless the type of infertility (e.g. both male and non-male factor infertility).

Wider implications of the findings
Based on these results, it is not recommended to decrease oxygen tension below 5% during culture, as this does not improve fertilization and euploidy rates. Nevertheless, embryo quality and blastocyst formation was found to be higher in 5% Group and this findings should be confirmed in a larger sample size.

Trial registration number
not applicable