P–526 Incidence of mosaic embryos in day–6 blastocysts, may late blastulation predispose to mosaicism?

Abstract Study question May the mosaicism ratio be influenced by the time of blastulation in preimplantation genetic testing for aneuploidies (PGT-A)? Summary answer The mosaicism ratio is significantly higher in day–6 blastocysts when compared to day–5 when transferable embryos are considered only (euploids and mosaics). What is known already Conventionally, PGT-A has classified preimplantation embryos as either euploid or aneuploid. Yet, a major improvement in PGT-A methodology, with the introduction of high sensitivity diagnostic Next Generation Sequencing (NGS) technology, has allowed the identification of a third embryo category: the mosaic (Coll et al., 2021). Embryonic mosaicism can be defined as the presence of karyotypically distinct cell lines within an embryo and can be detected by NGS at a rate between 20–80%. In the absence of euploid embryos, mosaic embryos, when transferred, have been shown to deliver healthy live births (PGDIS, 2019). Study design, size, duration This retrospective study was based on 9828 trophectoderm biopsies performed in a single ART clinic between January 2017 and October 2020 for PGT-A cycles with more than one blastocyst. PGT-A cycles with only one blastocyst were excluded because in these cycles’ day–5/day–6 biopsy percentage cannot be calculated. A total number of 8398 and 1430 blastocysts were biopsied on day–5 and day–6, respectively for PGT-A by ReproSeq on Ion Torrent S5 (Thermo Fisher Scientific). Participants/materials, setting, methods Three categories were defined in the PGT-A group with >1 blastocyst biopsied to compare the rate of mosaicism: C1:cycles in which blastocysts were only biopsied on day–5 (n = 1872), C2:99–60% of blastocysts were biopsied on day–5 (n = 483) and C3:0–60% of blastocysts biopsied on day–5 (n = 411). The mean female age (C1:36.0±5.2; C2:35.7±4.8; C3:37.1±4.9) and metaphase II oocytes punctured (C1:9.8±6.5; C2:10.4±5.2; C3:8.4±5.4) were similar and statistically non-significant in all groups. T-test and Chi-square tests were used where appropriate. Main results and the role of chance Overall, from the blastocysts biopsied on day–5 and 6, 35.4% and 25.5% were euploid, 13.7% and 14.7% were mosaic, 50.9% and 59.8% were aneuploid, respectively (p