L26/O-028 Paired trophectoderm and ICM analysis redefines segmental aneuploidy and embryo usability in poor-prognosis IVF patients

Abstract Study question What is the parental origin, timing, and distribution of segmental aneuploidy diagnosed as non-mosaic on a single trophectoderm biopsy, and how does re-biopsy refine usability? Summary answer Segmental aneuploidy diagnosed as non-mosaic is frequently mosaic, often paternal or post-zygotic, and re-biopsy frequently refines embryo usability. What is known already Segmental aneuploidy (SA), defined as gain or loss of sub-chromosomal regions, is commonly detected during preimplantation genetic testing for aneuploidy (PGT-A) and has been associated with adverse reproductive outcomes. SA may arise during meiosis or post-zygotic mitotic divisions, with re-biopsy studies indicating frequent discordance between embryonic samples. In contrast to whole-chromosome aneuploidy, SA shows a weaker association with maternal age and a notable paternal contribution. With the widespread adoption of next-generation sequencing for PGT-A, SA is increasingly reported; however, its parental origin, timing, and distribution within embryos particularly when classified as non-mosaic remain incompletely characterised. Study design, size, duration This retrospective cohort study analysed 105 donated human blastocysts from PGT-A cycles in which a non-mosaic segmental aneuploidy was identified on the initial trophectoderm biopsy. Embryos were recruited following initial PGT-A diagnosis, subsequently warmed, and re-biopsied. For each embryo, three paired samples were analysed. All procedures were conducted under IRAS ethical approval and an HFEA research licence. Participants/materials, setting, methods Blastocysts were donated by consenting patients undergoing clinical PGT-A at a single UK centre between 2020-2025. For each embryo, the original trophectoderm biopsy (TE1), a second trophectoderm biopsy (TE2), and a microdissected inner cell mass (ICM) were analysed. Low-pass whole-genome sequencing with SNP-based haplotyping, using parental DNA obtained from swabs, was performed to assess copy-number changes, mosaicism, and parental origin of segmental abnormalities. Main results and the role of chance A total of 105 embryos from 86 couples were analysed, comprising 315 paired biopsies (TE1, TE2, and ICM). Despite initial classification as non-mosaic on a single trophectoderm biopsy, 87.6% of embryos showed discordant segmental findings across samples, consistent with widespread mosaicism, while only 19.0% were fully concordant. Segmental abnormalities most commonly affected a single chromosome (77.1%), with 228 segmental events involving 135 chromosomes. Parental origin analysis demonstrated a higher paternal than maternal contribution (54.3% vs 42.9% of embryos with ≥1 event), although this difference was not statistically significant (P = 0.098). Evidence supporting a meiotic origin was identified in 45.3% of embryos. Post-zygotic dynamics were frequent, with rescue observed in 28.7% of affected chromosomes and intrachromosomal effects in 40%. Reciprocal gain and loss patterns with shared breakpoints occurred in 22.9% of embryos. Paired comparison of the second trophectoderm biopsy with the ICM altered the inferred chromosomal status in 37.1% of embryos, identifying a subset with concordant euploid or mosaic ICM findings, supporting potential clinical usability following appropriate genetic counselling. Limitations, reasons for caution The study is retrospective, single centre, and limited to embryos not transferred, precluding correlation with clinical or postnatal outcomes. Technical constraints of haplotyping limit parental origin assignment for small segmental events. Findings may not be generalisable to embryos without segmental abnormalities or to other testing platforms. Wider implications of the findings These findings challenge reliance on single trophectoderm biopsy for interpreting segmental aneuploidy and support viewing segmental abnormalities as biologically distinct from whole-chromosome errors. Incorporating parental origin, repeat biopsy, and cautious counselling may improve clinical decision making and prevent unnecessary embryo discard. Trial registration number No