Umbilical amino acid concentrations in appropriate and small for gestational age infants: a biochemical difference present in utero.

Plasma amino acid concentrations were determined in 28 pregnant women and their infants at term. Samples were obtained from 17 appropriate for gestational age and eight small for gestational age infants at cesarean section, while three small for gestational age fetuses were studied in utero by transabdominal cord sampling by means of ultrasonic guidance. Small for gestational age fetuses have significantly lower concentrations of alpha-aminonitrogen, compared with those of appropriate for gestational age fetuses, in both the umbilical artery and vein. Most of the difference is accounted for by the branched chain amino acids valine, leucine, and isoleucine. In contrast, hydroxyproline concentration is significantly higher in both the umbilical artery and vein of small for gestational age fetuses. The sum of the branched chain amino acid concentrations in the umbilical vein is directly related to maternal arterial values in both appropriate for gestational age and small for gestational age fetuses. Maternal arterial concentrations were slightly lower in small for gestational age fetuses and the regression analysis of umbilical venous versus maternal arterial branched chain amino acid concentrations was significantly different for small for gestational age and appropriate for gestational age infants. Umbilical venoarterial concentration differences in normal fetuses are significantly positive for most essential amino acids and for total alpha-aminonitrogen. In contrast, these differences were significant only for four essential amino acids in small for gestational age infants, while the total alpha-aminonitrogen venoarterial difference was not significant. The data obtained by transabdominal cord sampling from relatively undisturbed fetuses were in agreement with the data obtained at cesarean section; this information suggests that these differences between small for gestational age and appropriate for gestational age infants reflected steady-state conditions.