During pregnancy, growth of the foetus depends on an adequate glycine supply because it is needed for synthesis of fetal DNA, collagen and serine. Since pregnant adolescent girls give birth to lower birth weight babies, it is possible that they do not produce sufficient glycine to meet overall demands as their adult counterparts, especially after an overnight fast. The objective of the study was to measure and compare the flux of glycine among adolescents and adult women in the first and third trimesters of pregnancy. Glycine flux was measured by continuous intravenous infusion of 2H2-glycine in eight overnight fasted adolescents and in eight adult women in the first and third trimesters of pregnancy. There was a significant interaction between subject's age and time of pregnancy (P = 0·02), as weight-specific glycine flux decreased by 39 % from trimesters 1 to 3 in the adolescents but increased by approximately 5 % in the adults. Whole body glycine flux also decreased significantly in the adolescent group (P < 0·05) from trimesters 1 to 3, and this was associated with a significant reduction in plasma glycine concentration. In trimester 3, there was a positive correlation between glycine flux and the subject's age indicating that younger subjects had slower fluxes. These findings suggest that after a brief period of food deprivation, the pregnant adolescent cannot maintain glycine production as her adult counterpart in late pregnancy. It is possible that this inability to maintain endogenous glycine production makes her foetus more vulnerable to impaired growth if food deprivation becomes more frequent or is prolonged.