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Abstract Study question Which are the main patient, embryonic and laboratory variables associated with a diploid result in abnormally fertilized embryos? Summary answer Embryo diploid constitution is associated with maternal age, trophectoderm and inner cell mass quality as well as pronuclear (PN) number predicted by time-lapse incubator. What is known already In conventional in vitro fertilization cycles, morphological PN evaluation can identify proper fertilization and predict embryo ploidy constitution. Atypical PN patterns like 0PN, 1PN or more than 2PN are associated with ploidy abnormalities. However, we have previously shown that PN number is insufficient to predict embryo ploidy composition, and advanced single nucleotide polymorphism (SNP)-based methodologies for preimplantation genetic testing (PGT) detect a significant number of genetically diploid embryos from presumed abnormally fertilized oocytes. In this study, we characterized the diploid rescue rate associated with abnormal PN configurations, uncovering the link between ploidy results and relevant patient, embryonic and clinical/laboratory parameters. Study design, size, duration Retrospective observational study evaluating the association between diploid rescue rate of abnormally fertilized embryos and different patient, embryonic and laboratory features. Embryos were cultured in conventional or EmbryoScope™ time-lapse incubator (Vitrolife) until blastocyst stage and classified according to Gardner embryo grading system. Ploidy assessment was performed on 343 euploid trophectoderm (TE) biopsies collected between January 2021 and November 2023. Results were stratified according to patients age, embryo quality parameters and different laboratory procedures. Participants/materials, setting, methods Multi-center international study involving consenting patients using autologous gametes and undergoing PGT for aneuploidies (PGT-A) combined with ploidy analysis. Ploidy protocol was performed using a validated targeted NGS approach composed of a custom panel of 357 SNPs and a proprietary pipeline of analysis. Reliable ploidy classification was obtained using a combined algorithm strategy based on SNPs allele frequencies. Categorical variables were expressed in percentages. Nominal variables were analysed using the Cochran-Armitage test to detect trends. Main results and the role of chance Among 343 euploid embryos, diploidy rates obtained for 0PN, 1PN, 2.1PN and 3PN groups were 86.8% (n = 118/136;95%CI=79.89-91.96) 43.7% (n = 59/135;95%CI=35.19-52.50), 65.0% (n = 13/20;95%CI=40.78-84.61) and 26.9% (n = 14/52;95%CI=15.57-41.02) respectively. Regarding patient parameters, advanced maternal age, but not paternal age, was significantly associated with lower diploid rescue rates, with a decreasing trend from 72.5% (n = 79/109;95%CI=63.10-80.60) below age 35 to 42.9% (n = 30/70;95%CI=31.09-55.25) above age 40 (p < 0.01). When considering embryo morphological evaluations, Cochran Armitage test revealed a significant link between diploidy rates and TE quality with a decreasing trend going from 72.1% (n = 80/111;95%CI=62.76-80.17) to 52.7% (n = 79/150;95%CI=44.36-60.87) and 45.8% (n = 22/48;95%CI=31.37-60.83) for embryos classified with TE grade as A, B, and C, respectively (p-value<0.01). Similarly, a decreasing trend was reported in relation to inner cell mass (ICM) quality, when moving from grade A (67,1%, n = 94/140;95%CI=58.23-75.33) to grade C (54.8%, n = 17/31;95%CI=36.03-72.68) (p = 0.03). No clear association was reported for embryo expansion degree. Finally, among laboratory variables (oocyte freezing procedure, fertilization method, incubator type), only time-lapse significantly impacted expected diploid results. Overall, if abnormal PN number is confirmed by time-lapse (n = 42 samples), the accuracy in predicting ploidy abnormalities increased from 29.2% (n = 82/281;95%CI=23.93-34.87), in conventional incubators, to 83.3% (n = 35/42;95%CI=68.64-93.03) (p < 0.01). Consequently, expected diploid rescue rate decreased from 65.5% (n = 184/281;95%CI=59.60-71.03) to 16.7% (n = 7/42;95%CI=6.97-31.36) (p < 0.01). Limitations, reasons for caution Parental DNA analysis should be included to rule-out the residual risk of uniparental disomy in diploid embryos. The multi-center nature of the study didn’t support a uniform PN annotation, especially for centers lacking time-lapse observation. Future studies should focus on the clinical follow-up after the transfer of rescued euploid-diploid embryos. Wider implications of the findings We believe we reported the first evidence of maternal age effect on ploidy results in the largest dataset of abnormally fertilized embryos. We revealed a link between ploidy constitution and blastocyst morphological quality, providing important information for patient counselling. Time-lapse incubators offer a more accurate identification of abnormally fertilized oocytes. Trial registration number n/a