To define the normal ranges of umbilical cord blood oxygen saturation (SaO2) and acid-base status at birth and to evaluate the effect of gestational age on cord blood values in vigorous newborn infants following spontaneous vaginal birth from a vertex position. Prospective study. Department of Obstetrics and Gynaecology, University of Graz, Austria. Cord blood samples from 1281 vigorous newborn infants. Cord blood sampling was performed following on newborn infants following spontaneous vaginal birth in a vertex position. SaO2 was measured directly by a spectrophotometer and pH, base excess, pCO2 and pO2 by a pH/blood-gas analyser. Infants with a 5-minute Apgar score > or = 7 were considered vigorous. Subgroups were classified according to the gestational age: preterm, term and post-term (< 37, 37-42 and > 42 weeks, respectively). The median umbilical artery SaO2 was 24.3% and the 2.5th centile was as low as 2.7%. The median umbilical artery values were pH = 7.25, base excess = -4.3 mmol/L and pO2 = 16 mmHg. The 2.5th centiles were 7.08, -11.1 mmol/L and 5 mmHg, respectively. The median umbilical artery pCO2 was 50 mmHg and the 97.5th centile was 75 mmHg. The mean umbilical artery and vein SaO2 values were not significantly influenced by gestational age. The umbilical artery SaO2 and base excess values were strongly skewed. The mean umbilical artery pH values in preterm infants were higher than in other subgroups. The mean umbilical artery and vein base excess values were lower in post-term newborn infants than in other subgroups. The physiological range of oxygen saturation in umbilical cord of vigorous newborn infants at birth is wide and skewed. In contrast to pH and base excess, umbilical cord blood oxygen saturation is not influenced significantly by gestational age at birth.