Objective: To evaluate the fetal renal maturation by assessment of amniotic fluid microproteins and to show these proteins originate from fetal urine. Study design: Amniotic fluid proteins (total protein, albumin, high molecular weight protein-HMWP, low molecular weight protein-LMWP, alpha(1)-microglobulin and beta(2)-microglobulin) were determined in 39 pregnant women at delivery and by amniocentesis in 30 pregnant women. These values were compared with first urine values of neonates with the same gestational age. Results: Albumin was the largest protein component in the amniotic fluid. LMWP showed an increase in the amniotic fluid until the end of the second trimester; and as pregnancy advanced a progressive decrease occurred in parallel to fetal renal maturation. After 26 weeks' gestation, a strong correlation was identified between LMWP levels and alpha(1)-microglobulin, and between LMWP and beta(2)-microglobulin. No significant difference was detected between LMWP levels in the first urine of the neonates and in amniotic fluids. Conclusion: Microproteins in the fetal urine are of fetal origin. Fetal renal maturation can be evaluated by measuring microproteins in the amniotic fluid. Fetal renal maturation is best reflected by alpha(1)-microglobulin. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.