The distribution of gas values of umbilical cord blood was studied in an university clinic population during a period of 3 yr. All patients in labor were guided by obstetrical personnel and continuous electronic fetal monitoring (CTG). Microblood analyses were performed on indication of the CTG. The tenth percentile for the total population for umbilical artery blood pH was 7.14 and base excess was -12.7 mmol/l. The tenth percentile for pH in umbilical venous blood was 7.23 and base excess was -10.2 mmol/l. Increase of acidemia was seen in the following order: optimal pregnancy and labor, spontaneous vertex delivery, multiparity, primiparity, instrumental and breech delivery. Percentiles of umbilical cord blood gases could serve as an index for the standard of obstetrical care in addition to perinatal mortality and other measures of perinatal morbidity.