The maternal-fetal relationship of metabolic substrates was examined in 12 mothers undergoing cordocentesis for clinical reasons between 23 and 36 weeks’ gestation. Twenty cordocentesis procedures were performed. Blood glucose, insulin, β-hydroxybutyrate, glycerol and lactate were measured in simultaneously obtained maternal and fetal samples. There was a linear correlation between fetal and maternal concentration of glucose (y = 0.687x + 0.756, R2 = 0.65 and p = 0.001), and β-hydroxybutyrate (y = 0.443x + 0.16, R2= 0.938 and p = 0.0001). No relation between fetal insulin and glucose concentration was observed. No correlation was seen between maternal and fetal lactate concentration. The data on glycerol could be divided into two groups. When the fetal glycerol levels were below 100 μmol/l, the fetal glycerol concentration was always less than the simultaneously obtained maternal level. However, when the fetal glycerol levels were greater than 100 μmol/l, the corresponding maternal levels were lower than that in the fetus. The exact mechanism or significance of higher fetal glycerol levels remains unknown. These data demonstrate the usefulness of cordocentesis in the understanding of fetal metabolism. In correlation with isotopic tracer, cordocentesis could provide detailed insight into human fetus in the future.