Intervillous, umbilical venous and umbilical arterial blood samples were obtained by cordocentesis or fetoscopically from 200 pregnancies at 16–38 weeks gestation. The fetuses were either not affected by the condition under investigation or it was one which would not affect blood gas and acid-base status. Blood pH, pO2 pCO2, bicarbonate, base excess and plasma lactate concentration were determined and ranges for each parameter are presented. Samples obtained fetoscopically were more acidotic, hypercapnic and had a greater base deficit than those obtained by cordocentesis. In the umbilical venous samples, the pCO2 decreased, while the bicarbonate, base excess and lactate increased with gestation. In the umbilical arterial samples the pO2 decreased and pCO2 increased; there were no other significant changes with gestation. Similarly, in the intervillous samples the only significant change with gestation was that of a ecrease in pO2·