Amino acids in amniotic fluid: changes in concentrations during the first half of pregnancy.

Abstract

Extract: The free amino acids in amniotic fluid were quantitated by ion-exchange chromatography in human fetuses of 48− to 140-day gestation (mean, 91 days). Gestational age was estimated by crown-rump measurement. The amino acid pattern was more similar to that found in normal plasma than to that usually observed in urine. Fourteen of 26 amino acids (lysine, leucine, phenylalanine, isoleucine, valine, ornithine, alanine, tyrosine, methionine, arginine, serine, histidine, proline, and taurine) showed a statistically significant change in concentration (P < 0.005) which correlated inversely with fetal age (Fig. 1). Alanine had the highest average concentration during early gestation, 0.437 mM. The concentration of lysine showed the highest degree of correlation with fetal age (P × 0.001), with a correlation coefficient of 0.89. The most highly correlated single-regression curve was for lysine; where [lys] = 0.796–0.0052 (fetal age). None of the amino acid concentrations increased during the period of observation. Speculation: It has been suggested that the concentrations of constituents in amniotic fluid may serve as an indicator of fetal genotype and be of value in antenatal diagnosis. Because of the magnitude of the changes in amino acid concentration in amniotic fluid that occur during early gestation, accurate methods of fetal dating will be necessary if amino acid concentration is to be used for antenatal diagnostic purposes. The results of this study would suggest that regression analysis of amino acid concentration in amniotic fluid may prove to be an independent and reliable estimator of fetal age.