O-302 Triploid conceptions are predominantly caused by female meiosis II errors and their risk increases with advancing maternal age

Abstract Study question What are the incidence and origin of ploidy abnormalities in embryos derived from 2 pronuclei (2PN) zygotes, and is their risk linked to maternal age? Summary answer While embryo haploidy is usually due to male meiosis errors, triploidy is mainly caused by female meiosis II errors, whose incidence increases with maternal age. What is known already Normal fertilization is denoted by the appearance of 2PN 16-18 hours after insemination. Deviation from 2PN is considered evidence of abnormal fertilization and ploidy anomalies. Nonetheless, even 2PN embryos can be diagnosed with ploidy abnormalities during preimplantation genetic testing (PGT). The incidence of 3PN has already been linked to advanced maternal age. However, no conclusive and well-powered studies have yet investigated the incidence, origin, and maternal age correlation of genetically-diagnosed ploidy abnormalities. A targeted NGS-based approach that simultaneously analyzes copy number and genotyping data provides a comprehensive ploidy status assessment and helps address numerous basic and clinical research questions. Study design, size, duration Retrospective study including 96,660 trophectoderm biopsies analyzed between 2020 and 2022 using a targetedNGS-based PGT platform. A total of 1,063 embryos carrying haploidy or triploidy were used to investigate maternal age correlation, and parental/meiotic origin of the anomaly. Of these, 57 embryos were from concomitant PGT-M (PGT for monogenic disorders) cycles where also parental DNA was available. These trios (embryo-parents) were used to independently estimate parental/meiotic contribution to the ploidy abnormality and genome-wide recombination events. Participants/materials, setting, methods Targeted-NGS’s accuracy in genotyping and copy number (CN) detection were previously validated on triploid cell lines and multifocal blastocyst biopsies with known ploidy status derived from abnormally fertilized oocytes. Parental and meiotic origins were estimated using two independent approaches. First, they were inferred using sex chromosomes CN from 1,063 embryos with altered ploidy status. Secondly, they were directly calculated using genotyping data from 57 trios (embryo and parents) from PGT-M cycles. Main results and the role of chance The prevalence of ploidy abnormalities in 2PN-derived embryos was 1.1% (n = 1,063/96,660), with triploids accounting for 83.0% (n = 882/1,063) and haploids for 17.0% (n = 181/1,063). Remarkably,