The metabolic and endocrine milieu of the human fetus and mother at 18-21 weeks of gestation. I. Plasma amino acid concentrations.

Plasma levels of 17 amino acids were measured in 10 conscious mothers and their minimally stressed fetuses (mean body weight 302 g) at 18-21 wk gestation. Simultaneous blood samples were taken from the maternal antecubital vein, and from the fetal umbilical vein and artery by fetoscopy prior to termination of pregnancy. The mean concentrations of all amino acids were significantly higher in the fetal (2.26 mmol/liter) than in the maternal (0.96 mmol/liter) circulation, the total molar concentration of amino acids was 2.4 times greater in fetal than in maternal plasma. There was a significant positive relationship between maternal and fetal levels for most amino acids; the mean umbilical concentration difference was significantly positive for glycine, alanine, isoleucine, leucine, phenylalanine, and histidine.