With these limitations in mind, our group has recently completed a study utilizing embryos intended for clinical use (Copernicus IRB: RMA-2016-02). In this study, we enrolled 60 patients and split all ongoing embryos on day 3 of development to culture in either 2% or 5% oxygen until the blastocyst stage. The results have not yet been published, but culture in 2% oxygen after day 3 produced more blastocysts than culture in 5% oxygen. In total, 30 patients had more embryos reach blastocyst in 2% oxygen, while only 17 patients had more embryos reach blastocyst in 5% oxygen. Ten patients had an equal number in both conditions (3 withdrew from the study). This difference reached statistical significance.