During pregnancy, adult women with a normal BMI synthesise extra amino acids after an overnight fast by increasing body protein breakdown and decreasing amino acid oxidation. It is not known whether adolescent girls can make these adaptations during pregnancy. The present study aimed to measure and compare the protein, glutamine and alanine kinetics of adult women and adolescent girls at early-, mid- and late-pregnancy. Kinetics were measured in the overnight fasted state using intravenous infusions of 13C-leucine, 15N-glutamine and 15N-alanine in ten adults and twenty adolescents aged 14–17 years in the first and second trimesters (phase 1 study) and infusions of 13C-leucine and 15N2-urea in ten adults and eleven adolescents aged 16–17 years in the first and third trimesters (phase 2 study). In phase 1 study, there were no significant differences between the groups with regard to any of the kinetic parameters measured. In both groups, leucine flux increased (P< 0·05), the percentage of leucine flux oxidised decreased (P< 0·05) and non-oxidative leucine disposal to protein synthesis increased (P< 0·05) from the first to the second trimester. In phase2 study, leucine flux was significantly slower (P< 0·05) in the adult group than in the adolescent group during both trimesters, and whole-body leucine flux and non-oxidative leucine disposal increased significantly in the adolescent group (P< 0·05, respectively) and were higher in the adult group from the first to the third trimester. These results suggest that similar to their adult counterparts after an overnight fast, adolescent girls with a normal BMI provide extra amino acids required for net protein deposition during pregnancy by increasing protein breakdown and decreasing amino acid oxidation.