Fawzy M, et al. – The impact of culturing human embryos in vitro in 3.5% oxygen (O2) concentration is assessed in this study. Culturing human embryos, continuously from day 0 to 5 or 6, in 3.5% O2 concentration is related to majorly lower blastocyst formation rate and clinical outcomes parameters, but rather with significantly higher rates of fertilization and cleavage. Whether these outcomes hold true for other patient populations and culture media brands are still not known.