Plasma amino acids in appropriate- and small-for-gestational-age fetuses.

The fetal plasma amino acid profiles of 28 small-for-gestational-age fetuses were established in umbilical venous blood samples obtained by cordocentesis and the values were compared with those of 62 appropriate-for-gestational age fetuses. In the appropriate-for-gestational-age group fetal levels were significantly correlated with but were higher than the maternal levels, which indicates that amino acids are actively transported by the placenta. In the small-for-gestational-age group, the fetal plasma concentration and the fetomaternal ratio of essential amino acids was decreased and this decrease was significantly correlated with the degree of fetal hypoxemia. There was a variable response (e.g., alanine was increased, whereas serine and tyrosine were decreased) involved with nonessential amino acids. In umbilical venous hypoxemia the plasma ratio of nonessential/essential amino acids was increased, and the maternal plasma concentrations of both essential and nonessential amino acids were increased. These findings suggest that in uteroplacental insufficiency there is intrauterine starvation.