This study evaluated the effects of labor pain on maternalarterial hemoglobin oxygen saturation and neonatal acid-base status. Arterial oxygen saturation was monitorednoninvasively by pulse oximetry during labor and delivery. The patients studied (N = 46) were divided into four groupsaccording to obstetric history (primiparas and multiparas)and pain management during labor (lumbar peridural anes-thesia versus meperidine and nitrous oxide). Nine patientsat term but not in labor served as controls. Decreases ofarterial oxygen saturation were related to both subjectivepain, reported by visual pain analog scales, and to neonatalacid-base status at delivery. All values are reported as mean+- standard deviation (SD). Primiparas with peridural anes-thesia showed significantly less decrease in arterial oxygensaturation (1.7 2 1.4%; P < .001; N = 151, superior scores onthe visual pain analog scale (3.5 f 2.01, and a significantlybetter neonatal acid-base status (pH 7.29 t 0.06; P = .01;base excess −6.4 f 2.2; P < .05) as compared withprimiparas treated with meperidine and nitrous oxide (Sa027.2 ? 3.9%; visual pain analog scale 7.1 f 1.2; pH 7.21 2 0.1;base excess −9.5 2 4.5; N = 16). In multiparas there was nostatistically significant difference in decrease of arterial ox-ygen saturation, visual pain analog scale, and neonatalacid-base status.