Giving supplementary oxygen during elective cesarean section (CS) under spinal anesthesia is known to increase oxygen transfer to the fetus, but is associated with an increase in lipid peroxidation in the mother and fetus. Supplemental oxygen does not improve fetal pH or oxygen indices during a prolonged incision-to-delivery interval. Few data, however, are available on potential benefits/harm of supplementary oxygen during emergency CS under regional anesthesia. In this prospective, randomized, double-blind study, the authors evaluated how supplemental oxygen (60%) given to the parturient through delivery affects fetal oxygenation and lipid peroxidation in the mother and fetus. Women in laborwith singleton fetuses, excluding those with intrauterine growth restriction, known as fetal abnormality or pre-eclampsia (n=832), were recruited from a large university hospital in China. Women were divided into those with and without suspected fetal compromise. Those who ultimately delivered by emergency CS under regional anesthesia (n=131; 16%) were considered eligible for analysis. After transfer to the operating theater, a 5-mL blood sample was drawn from all women, who were then randomized to breathe either pure air (21% oxygen; air group) or 60% inspired oxygen (oxygen group) supplied by a high-flow, Venturi-type facemask.Mode of anesthesia for CSwas spinal (62% of air group; 61% of oxygen group); epidural (38% of air group; 33% of oxygen group); or combined spinal-epidural (0% and 6%, respectively). Times from the start of oxygen administration to delivery, skin incision-to-delivery, and uterine incision-to-delivery were recorded. At delivery, a segment of umbilical cord was taken before the infant’s first breath and umbilical artery (UA) and vein (UV) blood samples were obtained. After oxytocin was administered, another 5mL of maternal blood was drawn. Fetal oxygen indices were measured by blood gas analyzer, and batch analysis by gas chromatography mass spectrometry for 8-iso-prostaglandin (8-isoprostane) was conducted onmaternal and umbilical cord blood samples. Data from 6 women were excluded due to technical difficulties, leaving 125 patients for analysis (37 with suspected fetal compromise and 88 without). Maternal and fetal characteristics were similar between groups, as were duration of labor, skin incision-to-delivery, and uterine incision-todelivery intervals, and incidence of postspinal hypotension. Apgar scores were also similar with all scores >7 at 5 minutes. There were greater values for UA PO2 in the oxygen group compared with the air group (mean 2.2±0.5, vs. 1.9±0.6 kPa; P=0.01), in UA O2 content (6.6±2.5, vs. 4.9±2.8mL/dL; P=0.006), in UV PO2 (3.8±0.8, vs. 3.2±0.8 kPa, P<0.0001), and UV O2 content (12.9±3.5, vs. 10.4±3.8ml/dL; P=0.001). There were no differences between groups in maternal, UA, or UV 8-isoprostane blood concentration or blood pH. For women without suspected fetal compromise, the difference between the oxygen and air groups forUVPO2 was 18% and for oxygen content, 21%. In those with suspected fetal compromise, differences between oxygen and air groupswere 17%whenUVPO2was compared and 33% when oxygen content was compared. These cases had lower UA pH when compared with the group without suspected fetal compromise, but there were no differences in 8-isoprostane concentrations. These results suggest that administering 60% supplementary oxygen during emergency CS under regional anesthesia increases oxygen indices in umbilical blood without increasing lipid peroxidation in mother or fetus. The benefits are even greater in cases of fetal compromise. Hence, the authors believe that using supplementary oxygen under these circumstances has potential benefit in terms of improved fetal oxygenation without harm from lipid peroxidation.