In 29 pregnancies complicated by maternal diabetes mellitus paired samples of umbilical venous blood were obtained at cordocentesis and delivery to investigate the effect of delivery on indices of fetal oxygenation. After delivery by elective Caesarean section the median umbilical venous blood pH was significantly lower than predelivery, however, there was no significant change in the median umbilical venous blood pO2 or plasma erythropoietin concentration. In those delivered after labour the median umbilical venous blood pH and pO2 were significantly lower and the median plasma erythopoietin concentration was significantly higher than predelivery. The data of this study demonstrate that umbilical venous blood pH and pO2 and plasma erythropoietin are influenced by the mode of delivery and results obtained at delivery may not accurately reflect in utero homeostasis.