P-137 pronuclear characteristics and chromosomal constitutions of mono-pronuclear zygotes

Abstract Study question Limited studies have explored the correlation between the pronuclear characteristics, developmental potential, and genetic constitution of mono-pronuclear (1PN) zygotes. Summary answer Those 1PN zygotes developed into blastocysts have unique pronuclear characteristics and significantly higher rate of diploidy euploidy in comparison with the arrested embryos. What is known already It has been reported that 1PN blastocysts have similar morphokinetic features to two-pronuclear (2PN) embryos. Cytogenetic analyses reveal that some of the 1PN embryos have a normal chromosomal constitution. Study design, size, duration This retrospective cohort study included the 1PN zygotes from December 2021 to September 2022 (n = 388). Participants/materials, setting, methods Patients who underwent IVF or intracytoplasmic sperm injection (ICSI) in the Women and Children’s Hospital of Chongqing Medical University were recruited. Only 1PN zygotes cultured in the time-lapse incubators from the oocyte retrieval day were included. The pronuclear characteristics, including the area and diameter of the pronuclear, the number of nuclei, and the distance between the pronuclear and the near-polar body (DPNP), and the genetic constitutions were investigated. Main results and the role of chance The overall blastocyst formation and good-quality blastocyst rates in 1PN zygotes were 22.94% and 16.24%, significantly lower than that of 2PN zygotes (63.25% and 50.23%, respectively, P = 0.000). Compared to arrested embryos, 1PN zygotes that developed into blastocysts showed significantly larger area (752.57 ± 131.53 vs. 653.65 ± 116.58, P = 0.000), longer diameter of pronuclear (30.10 ± 2.90 vs. 27.28 ± 2.83, P = 0.000), a greater number of nuclei (11.92 ±  4.06 vs. 7.72 ± 3.07, P = 0.000), and shorter DPNP (13.73 ± 8.65 vs. 24.20 ± 13.51, P = 0.000). Of the tested embryos, the diploidy euploidy rate was significantly higher in good-quality blastocysts in comparison with the arrested embryos (66.67% vs. 11.76%, P = 0.000), which was also significantly higher in IVF-1PN blastocysts than in ICSI-1PN blastocysts (75.44% vs. 25.00%, P = 0.001). Limitations, reasons for caution The correlation between pronuclear characteristics and genetic constitution was investigated only in limited embryos. The small sample size might weaken the conclusions. In addition, part of the 1PN blastocysts were tested by NGS, which has inherent limitations because it could not find the uniparental diploidy. Wider implications of the findings The 1PN blastocysts, especially from IVF-1PN zygotes, have high diploidy euploidy rates. The utility of 1PN blastocysts is potentially beneficial for patients without other viable embryos. Trial registration number Chongqing Science and Technology Committee (grant number: CSTB2022NSCQ-MSX0253), Chongqing Health Committee (grant number: 2021MSXM108), Yuzhong District Science and Technology Committee (grant number: 20190143), and Women and Children’s Hospital of Chongqing Medical University (grant number: 2021YJQN07).