Risk factors associated with meiotic errors in blastocysts with mosaic results on biopsy.

STUDY QUESTION: What are the risk factors associated with meiotic errors in blastocysts with mosaic biopsy results? SUMMARY ANSWER: Meiotic errors were identified in 3.9% of blastocysts with mosaic biopsy results and were correlated with high-level mosaicism, maternal origin, and advanced maternal age. WHAT IS KNOWN ALREADY: Chromosomal mosaicism mainly arises from postzygotic mitotic errors, except for rare events from rescued meiotic errors. Preimplantation genetic testing for aneuploidy origin (PGT-AO) can distinguish meiotic errors from mitotic errors; however, the clinical value of PGT-AO remains to be explored. STUDY DESIGN SIZE DURATION: A retrospective cohort of 391 blastocysts with mosaic biopsy results from a university-based fertility centre in China between January 2020 and December 2024 was analysed by PGT-AO. Risk factors associated with meiotic errors in blastocysts with mosaic biopsy results were explored. Pregnancy outcomes following 96 mosaic embryo transfers (METs) of embryos with mitotic errors were compared with those following transfer of 288 matched euploid embryos. Additionally, 7 donated blastocysts were separated for single-cell DNA sequencing. PARTICIPANTS/MATERIALS SETTING METHODS: The parental origin and cell-division origin were analysed in 391 blastocysts classified as euploid-aneuploid mosaic from a cohort of 8932 blastocysts detected using the single-nucleotide polymorphism (SNP) array. In the prospective study, pregnancy outcomes following METs of embryos with mitotic errors were compared with those following transfer of matched euploid embryos at a 1:3 ratio using propensity score matching. The primary outcome was the live birth/ongoing pregnancy rate (LB/OPR) of METs involving mitotic errors. Multivariate logistic regression analysis was used to evaluate risk factors for pregnancy outcomes. Prenatal and placental samples were analysed by SNP array and/or FISH for genetic verification. Single-cell DNA sequencing of 314 cells separated from 7 donated blastocysts (4 with high-level mosaicism, 2 with meiotic aneuploidy, and 1 with low-level mosaicism) was conducted to assess actual mosaicism. The chromosomal constitution of the blastocysts was comprehensively evaluated at the single-cell level, and concordance with the initial PGT-A results was assessed. MAIN RESULTS AND THE ROLE OF CHANCE: A SNP-based mosaicism quantification platform was established and validated using mixtures of single cells of varying ploidy to mimic clinical mosaic samples. In the retrospective cohort, the error origin was successfully determined for 384 of the 391 blastocysts with euploid-aneuploid mosaicism. The overall meiotic error rate was only 3.9% (15/384). Meiotic error was identified in 9.6% of blastocysts with mosaicism from women of advanced maternal age, which was significantly greater than the 3.0% observed in blastocysts with mosaicism from young women (OR = 3.43, 95% CI 1.12-10.46; P = 0.039). In addition, meiotic error was identified in 7.2% (10/139) of blastocysts with high-level mosaicism but 2.0% (5/245) of blastocysts with low-level mosaicism (OR = 3.72, 95% CI 1.25-11.12; P = 0.012). Meiotic error was also significantly greater in blastocysts with maternal-origin mosaicism than in those with paternal-origin mosaicism (8.9% vs 0.4%, OR = 21.66; P < 0.001). Clinical outcomes were comparable between METs and euploid embryo transfers, with no significant difference in LB/OPR (47.9% vs 52.8%, P = 0.409). Single-cell sequencing of donated blastocysts with high-level mosaicisms of mitotic errors demonstrated highly variable mosaic levels, ranging from a reproducibility rate of 8.7-100% for initial mosaic abnormalities at the single-cell level. LARGE SCALE DATA: Due to the individual privacy of the patients, the data are not publicly available. LIMITATIONS REASONS FOR CAUTION: Intrinsic technical noise cannot be completely distinguished from genuine mosaicism in the PGT-AO platform. The sample size of the MET cohort was limited, with few cases undergoing prenatal/postnatal genetic validation. In the single-cell study, the number of verified embryos was relatively small, and the threshold of copy number variation (CNV) detection was 10 Mb, which may have led to the underestimation of CNVs under 10 Mb. WIDER IMPLICATIONS OF THE FINDINGS: PGT-AO analysis demonstrated a low prevalence of meiotic errors in human blastocysts with mosaic biopsy results. PGT-AO is recommended for embryos from patients of advanced maternal age, embryos with high-level mosaicism, and embryos with maternal-origin mosaicism in PGT cycles. FUNDING: This study was supported by grants from the National Key Research and Development Program of China (No. 2023YFC2705503), National Natural Science Foundation of China (No. 82071716), Natural Science Foundation of Guangdong Province (No. 2025A1515010982), Key Clinical Technique of Guangzhou (No. 2023P-ZD19), and Medical Scientific Research Foundation of Guangdong Province (No. A2025203). DISCLOSURES: All authors declare no conflicts of interest.