Developmental trajectory of monopronucleated zygotes after in vitro fertilization when they include both male and female genomes

Objective: To examine the cause of monopronucleated zygote (1PN) formation that includes both maternal and paternal genomes. Design: Retrospective cohort study. Setting: Private fertility clinic. Patient(s): A total of 44 1PN and 726 2-pronuclear zygotes from 702 patients were observed using 2 different time-lapse observation systems. Intervention(s): Previously recorded time lapse data were reviewed to examine the mechanism of 1PN formation. Main Outcome Measure(s): The distance between the position of the second polar body extrusion and the fertilization cone or epicenter/starting position of the cytoplasmic wave was measured, and the consequent data were analyzed. Cytoplasmic waves were confirmed using vector analysis software. Result(s): The cut-off value for the difference in the distance between the position of the second polar body extrusion and the fertilization cone or the epicenter/starting position of the cytoplasmic wave was 17 μm (AUC: 0.987, 95% CI: 0.976–0.999) for the Embryo Scope and 18 μm (AUC: 0.972, 95% CI: 0.955–0.988) for the iBIS time-lapse observation systems. Conclusion(s): In this study, it was found with a high degree of accuracy that a monopronucleus is formed when the fusion of the sperm takes place within 18 μm from the point of the second polar body extrusion. The theoretical chance of 1PN occurrence after in vitro fertilization is 2.7% when the sperm is considered to be fused anywhere in the plasma membrane of an oocyte.