One area that has been subject to significant scrutiny is the relationship between incubator oxygen concentration and early embryonic development. Oxygen plays a central role in embryonic metabolism. The mechanism governing its utilization is dependent on the stage of embryonic development. During the first 3 days of development, oxygen reaches the embryo via passive diffusion and its concentration gradient is regulated by oxygen consumption during oxidative phosphorylation. Inefficiencies in this process - due to compromised integrity of the inner mitochondrial membrane or alterations in substrate availability - can result in excessive production of harmful reactive oxygen species which can cause significant damage to cellular machinery and ultimately lead to embryonic arrest.