Biochemical investigation of fetal renal maturation in early pregnancy.

The objective of this study was to evaluate the biochemical indices of normal fetal kidney maturation in early pregnancy. Urea, creatinine, gamma-glutamyltransferase, and beta 2-microglobulin levels were measured on paired samples of amniotic and coelomic fluids and maternal serum collected at the time of pregnancy termination in a group of woman with pregnancies between 8 and 11 wk, or on paired samples of amniotic fluid and maternal serum collected from woman undergoing early transabdominal amniocentesis between 11 and 16 wk. Before 11 wk of gestation (n = 12), significantly lower concentrations of creatinine and beta 2-microglobulin, and higher concentration of gamma-glutamyltransferase were found in amniotic fluid compared with both maternal serum and coelomic fluid. Significant positive correlations were found between gestational age (8-16 wk) and amniotic fluid (n = 47) levels of urea (r = 0.45; p < 0.01), creatinine (r = 0.68; p < 0.001), gamma-glutamyltransferase (r = 0.71; p < 0.001), and beta 2-microglobulin (r = 0.69; p < 0.001). The maternal serum levels of the corresponding variables did not varied significantly. The abrupt increase in creatinine, gamma-glutamyltransferase, and beta 2-microglobulin amniotic fluid concentrations after 10 wk of gestation provides biochemical evidence for the maturation of the fetal renal glomerular function at a time when the reabsorption capacity of the proximal tubular cells is not established. We suggest that this important variation in the amniotic fluid composition, unrelated to any comparable changes in the maternal serum, reflects the fetal kidney development from the mesonephros to the metanephros.