Abstract Study question Is it possible to determine diploid mono-pronuclear (1PN) human embryos by the speed of development to blastocyst formation? Summary answer The developmental time to reach the blastocyst stage is a discrimination indicator for diploid 1PN embryos. What is known already We have previously reported that 80.7% of 1PN zygotes had a biparental chromosome using a Live-Cell imaging technique, and some of these developed to the blastocyst stage (Tokoro et al. ASRM 2013). Furthermore, we have reported that these blastocysts can result in a healthy live birth (Tsuji et al. ASRM 2020), and there was no effect on growth up to 5 years of age (Tsuji et al. ASRM 2023). On the other hand, it was also shown that a few of the 1PN embryos that developed into blastocysts were haploid embryos. Study design, size, duration This was a retrospective study that included 28 blastocysts derived from a 1PN zygote. The time period was 72 months (January 2017 to December 2022). These embryos are used for research following obtaining informed consent. Participants/materials, setting, methods The frequency rate of diploid embryos was compared from time of PN fading to blastocyst formation (Gardner’s classification: ≥ expansion grade 3). Two development time range groups were examined and designated: fast-developing embryos (65.7-78.7h) and slow-developing embryos (116.8-139.7h). Ploidy status was determined using whole blastocyst cells and NGS-based SNP analysis. Statistical significance was determined using Fisher’s exact test (level of P<0.05). Main results and the role of chance There was no significant difference in the average age (years) of the two development time groups (fast-developing embryos group; 35.5 +/- 5.5, slow-developing embryos group; 34.1 +/- 2.8). The frequency of diploid status in 1PN embryos with fast developing embryos was 100% (14/14), and in 1PN embryos designated as having slow development, the frequency of diploid embryos was only 14.3% (2/14), haploid embryos 71.4% (10/14), and other was 14.3% (2/14). As a result, the diploid embryo rate was significantly higher in fast-developing embryos than in slow-developing embryos (P<0.001). Limitations, reasons for caution This study was limited to extremely fast or slow-developing embryos to see trends. It is important to study the threshold time between diploidy and haploidy in 1PN embryos. Wider implications of the findings These results confirmed that fast developing 1PN embryos have a high diploid rate and can be used as transferable embryos in addition to 2PN embryos. Conversely, slow developing 1PN embryos should be considered to have a lower priority for transfer because of the high rate of haploid embryos. Trial registration number not applicable