We read with interest the Clinical Opinion of Hamel et al in which the authors suggest that supplemental maternal oxygen should only be used for conditions of maternal and not fetal hypoxemia. Although the article raises important points, we believe there are indications of maternal oxygen for treatment of fetal hypoxemia. The authors note 2 randomized studies, containing relatively low numbers of laboring patients, which failed to demonstrate a fetal benefit. Notably, both studies reported no difference in umbilical cord pH among the groups. Importantly, pH is influenced by both metabolic and respiratory (ie, pCO2) acidosis, the latter of which is independent of O2 levels and may change acutely and significantly within minutes with cord occlusion. The authors’ further suggest that maternal oxygen supplementation in the presence of severe fetal acidosis may have potential detrimental effects (eg, free radical formation) and highlight the consequences of hyperoxia. However, one cannot produce fetal hyperoxia via maternal oxygen supplementation. Maternal O2 supplementation markedly increases maternal arterial pO2, but the minimal increase in arterial O2 content in normoxemic women results in a minimal impact on normoxemic fetal arterial O2 content. In contrast, under conditions of relative fetal hypoxemia, the fetus operates at the steep portion of the O2 saturation curve and achieves greater benefit of maternal hyperoxia. Late decelerations are believed to be secondary to either chemoreflex (vagal) responses to mild hypoxemia or direct myocardial suppressive effects of more severe hypoxemia. Variable decelerations result from umbilical cord compression-induced hypoxemia and result in progressively increasing level of acidosis dependent upon the severity and duration of the decelerations. As persistent hypoxemia results in acidosis, preventing mild or more severe hypoxemia from producing acidosis is the physiologic basis for oxygen supplementation. Although maternal hyperoxia is likely overutilized in labor management, we suggest that maternal oxygen supplementation should be continued in cases in which early fetal hypoxemia is suspected on the basis of fetal monitor tracings.