Can We Optimise The Time That We Perform The Fertilisation Check In The Lab? Lessons Learnt From Time-Lapse Incubation

Abstract Study question Can time-lapse data be used to identify the optimum time to perform the fertilisation check for oocytes cultured in standard incubation? Summary answer The optimum time to perform fertilisation checks for oocytes cultured in standard incubation is 16.5hpi+/–0.5h. What is known already Time-lapse incubation allows the embryologist to retrospectively review collated images of oocytes and embryos to capture important embryological observations that may have otherwise been missed. This is a luxury not available to embryologists when oocytes or embryos are cultured in standard incubation. Traditionally, the optimum time to perform the fertilisation check is 17 hours post insemination (hpi) +/- 1 hour. It was hypothesised that this could be fine-tuned ensuring the maximum number of fertilised oocytes were observed, thereby increasing the number of usable embryos for the patient. Study design, size, duration This was a retrospective, multicentre analysis including data from 27,022 ICSI derived embryos cultured in time-lapse incubation between January 2011 to November 2019. Participants/materials, setting, methods The time of pronuclei appearance and disappearance was recorded using the time-lapse incubation software. The number of oocytes exhibiting normal fertilisation (defined as the presence of two pronuclei) during 30 minute intervals from 15hpi to 20hpi was determined. Main results and the role of chance Between 15–17.5hpi the average number of oocytes exhibiting normal fertilisation was 98.19% with most oocytes having visible pronuclei at 16–16.5hpi (98.32%). At 18–18.5hpi the number of visible pronuclei falls to 95.53% and continues to fall to 87.02% at 19.5–20hpi meaning that over 3000 (11%) normally fertilised oocytes, within this cohort, would not be identified. Limitations, reasons for caution The conclusions of this investigation cannot be effectively extrapolated to IVF embryos as only ICSI embryos were used for the determination of the results. Wider implications of the findings: The optimum time to perform fertilisation checks for oocytes cultured in standard incubation is 16.5hpi+/–0.5h. However, without the use of time-lapse incubation, the fertilisation of at least 2% of embryos that create a fetal heart will be missed, even if the fertilisation check is performed in the optimum window (16.5hpi+/–0.5h). Trial registration number Not applicable