OBJECTIVE To estimate the acid-base and oxygen status of fetal blood and compare with maternal placental venous blood. DATA SOURCES AND CALCULATIONS: We selected pH and blood gas data from the literature pertaining to umbilical vein and artery blood obtained by cordocentesis and estimated values for the 30th and 40th gestational week. Average values for maternal venous blood leaving the placenta were estimated on the assumption of equal maternal arterio-venous and umbilical veno-arterial total oxygen concentration differences. RESULTS pH and pCO2 of maternal blood leaving the placenta and umbilical vein blood are almost identical at week 30. A small pCO2 and pH difference may exist at week 40. The pO2 of the maternal placental venous blood and umbilical vein blood are almost identical at week 30, but at week 40 a pO2 difference indicates an umbilical arterio-venous shunting of as much as 30%. The fetal mixed venous pO2 falls from 2.6 kPa to 2.2 kPa from the 30th to the 40th gestational week. CONCLUSIONS More accurate measurements are needed to confirm our results. Future measurements should be performed with a combined pH-blood gas analyser and haemoximeter to allow determination of the complete oxygen status of the blood.