Eighty fetuses referred because of maternal alloimmunization or maternal infection underwent umbilical venipuncture and were found to be unaffected. Norms for hematologic, biochemical, and pressure measurements were constructed from 18 weeks until term. Total red blood cell (RBC) count (p less than .0001), hemoglobin (p less than .0001), white blood cell count (p less than .0001), and platelet count (p = .04) and volume (p less than .03) each rose with advancing gestation. The number of nucleated RBCs declined significantly (p less than .0001). Umbilical vein carbon dioxide pressure rose (p less than .009) and oxygen pressure declined (p = .005) with advancing gestation. Total protein and albumen rose (each p less than .0001). Aspartate aminotransferase (p less than .02), alanine transferase (p less than .0001), and gamma-glutamyl transpeptidase (p less than .0001) each rose with gestation, independent of any other measured value. The rise in lactic dehydrogenase with gestation was found to depend on pH (p = .0002) rather than gestational length. Paired maternal-fetal serum samples were obtained from an additional 50 abnormal fetuses; there was no significant relationship between mother and fetus for the concentrations of any of the studied enzymes. Umbilical vein pressure rose progressively with gestational age (p less than .004). These studies reveal new information on fetal development and provide a baseline from which to evaluate either fetal disease or the response to therapy.