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 an d 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 amni otic cavity, placental tissue, decidua, and fetal blood of 30 early pregnan cies. RESULTS: Before 11 weeks' gestation, placental P-O2 was 2.5 times lower tha n decidual P-O2. The P-O2 increased independently at both sites during gest ation, but a P-O2 gradient of 13.3 mm Hg persisted during the fourth month. At 13 to 16 weeks, P-O2, oxygen saturation, and oxygen content gradients w ere observed between the fetal blood and the placenta and between the place nta and underlying decidual tissue. There was no fetoplacental gradient for pH and P-CO2 between 7 and 16 weeks, but fetal blood pH values were much l ower and fetal P-CO2 values were much higher than those reported in older f etuses. 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.