The tension of oxygen measured in the oviducts of several mammals was 5–8.7 %, but this drops in the uterine milieu to <2 % in cows and primates. For embryo culture in human in vitro fertilization (IVF), a non-physiologic level of 20 % oxygen has been used for the past 30 years. However, several animal studies have shown that low levels of oxygen plays an important physiological role in reducing the high levels of detrimental reactive oxygen species within cells, influences the embryonic gene expression, helps with embryo metabolism of glucose, and enhances embryo development to blastocysts. However, clinical studies have given contradictory results. Nevertheless, in nearly all reports, some kind of improvement has been observed, either in embryo development or in implantation and no detriments have been reported. For these reasons, more and more IVF laboratories utilize low oxygen during embryo culture.
