Umbilical amino acid concentrations in appropriate and for gestational age infants: a biochemical difference pres in utero Irene Cetin, MD, Anna M. Marconi, MD, Patrizia Bozzetti, MD, Lucia P. Sereni, MD, Carlo Corbetta, MD, Giorgio Pardi, MD, and Frederick C. Battaglia, MD Milan, Italy, and Denver, Colorado 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 a-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 a-aminonitrogen. In contrast, these differences were significant only for four essential amino acids in small for gestational age infants, while the total a-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. (AM J OesTET GYNECOL 1988;158:120-6.) Key words:Fetal metabolism, amino acid concentrations, umbilical vein, umbilical artery, intrauterine growth retardation, transabdominal cordsampling Studies in animals and humans have suggested that amino acids are major substrates for fetal energy pro- duction and growth. The umbilical uptake of amino acids has been measured in the fetal lamb under chronic steady-state conditions in late gestation. 1 The total fetal uptake of amino acids exceeded their net accretion rate during fetal life, an observation consis- tent with other data supporting the role of amino acids as fuels during fetal life.' 4 From the Perinatal Unit, First Institute of Obstetrics-Gynecology, University of Milano School of Medicine, the Laboratory of Met- abolic Diseases, Istituti Clinici Persezionamento, Milan, and the Division of Perinatal Medicine, Department of Pediatrics, Uni- versity of Colorado School of Medicine. This work was supported by Grant No. 85.00483.56 from the Italian National Research Council and by National Institutes of Health Center Grant No. HD-2076I and National Institutes of Health Program Grant No. HD-0078I. TableI. Birth weight-gestational age distribution of SGA infants Birth weight (gm) Gestational age (wk) Sampled at cesarean section 1950 37 2150 38 2400 38 2200 38 2500 39 2400 40 2100 39 2300 37 Transabdominal cord sampling 2500 39 1500 36 2800 39 % <2 <2 <5 <2 <2 <2 <2 <5 <2 <2 <10 Received for publication February 23, I987; revised June 8, I987; accepted August 1 I, 1987. Reprint requests: Giorgio Pardi, MD, Clinica Ostetrica e Ginecolo- gica, Universita degli Studi di Milano, Ospedale San Paolo, via A. Di Rudin£ 8, 20142 Milano, Italy. In the human being, as in most other mammalian species, the concentration of amino acids, including the. essential amino acids, is significantly higher in fetal cord plasma than in maternal plasma," 6 a finding consistent 120