Time-lapse videography of human embryos: Using pronuclear fading rather than insemination in IVF and ICSI cycles removes inconsistencies in time to reach early cleavage milestones

Time-lapse videography showed that human early cleavage embryos were quicker following intracytoplasmic sperm injection to reach developmental milestones compared to in vitro fertilization when using insemination as the timing start point (t0), due to differences in the time taken for embryos to reach pronuclear fading (PNF). These differences disappeared when PNF was used as t0. Using a biological rather than procedural t0 will allow a unified assessment strategy to be applied to all cycles irrespective of the insemination method.