Evaluation of respiratory gases and acid-base gradients in human fetal fluids and uteroplacental tissue between 7 and 16 weeks’ gestation

Objective: Our purpose was to evaluate the changes in intrauterine gases and acid-base gradients inside the human fetoplacental unit at 7 to 16 weeks’ gestation. Study Design: Respiratory gases and acid-base values were recorded by means of a multiparameter sensor and samples from inside the exocoelomic or amniotic cavity, placental tissue, decidua, and fetal blood of 30 early pregnancies. Results: Before 11 weeks’ gestation, placental PO2 was 2.5 times lower than decidual PO2. The PO2 increased independently at both sites during gestation, but a PO2 gradient of 13.3 mm Hg persisted during the fourth month. At 13 to 16 weeks, PO2, oxygen saturation, and oxygen content gradients were observed between the fetal blood and the placenta and between the placenta and underlying decidual tissue. There was no fetoplacental gradient for pH and PCO2 between 7 and 16 weeks, but fetal blood pH values were much lower and fetal PCO2 values were much higher than those reported in older fetuses. Conclusions: Early human placental tissue develops in a physiologically low-oxygen environment compared with uterine tissue. This may be necessary to allow specific placental metabolic activities and to protect both placental and fetal tissues against toxic oxygen metabolites. (Am J Obstet Gynecol 2001;184:998-1003.)