ASPECTS OF FETAL PHYSIOLOGY FROM 18 TO 37 WEEKS GESTATION AS ASSESSEDBY BLOOD-SAMPLING

Objective: To construct reference ranges for fetal pH, oxygen pressure (PO2), and hematologic and biochemical blood constituents, which can be used to analyze changes with gestation and differences with materna l values, thus elucidating some aspects of fetal biology and the effec ts of the maternal and placental environments. Methods: We assayed ven ous pH, PO2, hematocrit, glucose, uric acid, urea, creatinine, total p rotein, total and direct bilirubin, aspartate aminotransferase, alanin e aminotransferase, gamma-glutamyltransferase, alkaline phosphatase, l actic dehydrogenase; amylase, pseudocholinesterase, creatine kinase, t riglycerides, and cholesterol concentrations in 157 fetuses and 134 mo thers who underwent fetal blood sampling from 18 to 37 weeks' gestatio n. None of the fetuses was infected or had chromosomal, hematologic, o r hormonal abnormalities. Results: All the variables analyzed were sim ilar in fetuses sampled at the placental cord insertion (n = 125) or a t the intrahepatic vein (n = 32). Maternal and fetal concentrations of glucose (r = 0.79, P < .001), urea (r = 0.96, P < .001), creatinine ( r = 0.83, P < .001), and uric acid (r = 0.94, P < .001) correlated sig nificantly, and their differences exhibited significant changes: the m aternal-fetal differences of glucose and urea increased, whereas those of uric acid and creatinine decreased with advancing gestation. Fetal pH and PO2 decreased with gestational age, whereas hematocrit increas ed, similar to what has been described previously. All of the other va riables, with the exception of amylase and cholesterol, changed signif icantly during the investigated period of pregnancy. Gestational age e xplained at least 40% of the variance in values of fetal total protein , pseudocholinesterase, alanine aminotransferase, creatine kinase, and triglycerides, but only 3-25% of the variation in the remainder. Most enzymes were higher in the fetus than in the maternal circulation, an d all except alkaline phosphatase increased with gestational age. The maternal-fetal glucose difference correlated significantly with hemato crit, pH, and PO2, independent of gestational age and independent of e ach other. Conclusion: With the exception of aspartate aminotransferas e, all of the analyzed fetal variables were different from the materna l values, and most changed with gestational age. The mechanisms leadin g to these fetal specificities remain mostly uncertain, but the provis ion of reference ranges for several blood constituents may be useful i n the differential diagnosis of fetal disease.