What laboratory factors most strongly influence blastocyst formation rates and embryo quality?

[Verified against the Vienna Consensus 2017] Confounders the consensus names explicitly for blastocyst development rate: timing of laboratory observation, culture medium, and culture conditions — with pO2 concentration singled out. Oxygen tension deserves emphasis because it is the most consequential and the least visible. It is a facility-level parameter, not a per-cycle one, so a change affects every patient simultaneously and shows as a cohort-wide shift rather than individual poor outcomes. Other laboratory determinants to audit: Incubator temperature and gas calibration, and how often doors are opened — bench-level stability rather than set-point. Culture medium lot changes, and whether lot-to-lot quality control is performed. Consumable lot changes, particularly contact materials. Mouse embryo assay data should be on file. Air quality and volatile organic compounds. Personnel — technique variation between embryologists is real and is why individual-operator KPI tracking matters alongside laboratory-level figures. Workload and time pressure. Upstream factors that are not laboratory failures but present as them: Oocyte cohort quality, proxied by reference indicators. Fertilisation indicators — 1PN rate above 5% for IVF or 3% for ICSI specifically flags gamete handling or culture conditions. Cleavage rate below 95% competency, with the cohort effect noted in the consensus. Assessment artefact before intervention: confirm the denominator is 2PN oocytes and assessment is at 116 ± 2 h. Both errors are common and both change the apparent rate without anything changing in the incubator.