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The concentration of 23 free amino acids was measured in homogenates of fetal liver and samples of fetal plasma from 20 pregnancies between 12 and 17 weeks gestation and compared with those found in matched samples of maternal plasma and amniotic fluid. A feto-maternal plasma concentration gradient was observed for 21 amino acids indicating that the feto-maternal amino acid gradient across the placenta is established from very early in pregnancy. The amino acid concentration pattern was similar in fetal plasma and amniotic fluid but different in fetal liver, supporting the concept that it is essentially placental transport and metabolism that provides the fetus with these molecules. The highest amino acid concentration was found for glutamine in fetal plasma and glutamic acid in fetal liver. Very low concentrations of glutamic acid in fetal plasma suggest that this amino acid is actively taken up by the fetal liver. Citrulline, alpha-aminobutyric acid, methionine, arginine and tryptophan were not measurable in fetal liver tissue, indicating that this organ has a limited role in utero in the metabolism of these amino acids. Significant positive correlations were found between fetal plasma and amniotic fluid for the concentration of most amino acids whereas only the concentration of threonine was found to be positively correlated between fetal liver and plasma. These results suggest that during the second trimester passive diffusion through the unkeratinized fetal skin is the main pathway for amino acids between the fetal circulation and the amniotic cavity.

Free amino acids in human fetal liver and fluids at 12–17 weeks of gestation