Oxygen supplementation during general anesthesia for cesarean section (CS) has been shown to improve fetal oxygen delivery as well as neonatal outcomes, and has been a standard practice since the 1960s. The placenta is much less efficient than the adult lung at oxygen transfer. The oxygen content of blood delivered to the fetal heart (and pumped to the tissues of the fetus) is less than that leaving the placenta. This is due to mixture in the fetal inferior vena cava of relatively oxygenated blood from the ductus venosus (the major branch of the umbilical vein) and the relatively deoxygenated blood from the fetal portal sinus and the blood from the fetal lower body. In addition to the lack of evidence supporting the use of supplemental oxygen during elective CS performed under neuraxial anesthesia, there has been some recent suggestion of a possible detrimental effect of hyperoxia.