This study presents kinetic parameters of glycine metabolism during pregnancy and the influence of fuel availability in the maternal compartment. The effect of gestational diabetes mellitus (diet treated) and pre-gestational diabetes mellitus (diet and insulin treated), accompanied by increased fetal growth on [15N]glycine kinetic parameters, compared to normal pregnancies with normal fetal growth, has been determined. This process was measured by administration of a single-dose of [15N]glycine to post-absorptive normal and diabetic pregnant women during the third trimester of pregnancy. Gas chromatography-mass spectrometry was used to determine the 15N enrichment of plasma glycine, and to calculate the pool sizes, turnover rate constants, fluxes and metabolic clearance rates. Glycine pool sizes in pre-gestational diabetes were significantly larger than those in normal pregnancy and gestational diabetes. Glycine turnover rate constants and metabolic clearance rates were not significantly different between the normal pregnant women and the two diabetic groups of pregnant women. Glycine fluxes were significantly higher in the pre-gestational diabetic pregnant women than in gestational diabetes and normal pregnancy. The pre-gestational diabetic pregnant women delivered fetuses with higher birth weights than the other two groups.