Objective: This study was undertaken to determine amniotic fluid gas values and acid-base balance during maternal hyperoxemia and hypoxemia in early pregnancy. Study Design: Anesthetized sheep (n = 12) in early and mid pregnancy (0.3 to 0.5 gestation) were subjected to hyperoxemia followed by hypoxemia. Amniotic fluid PO₂, pH, PCO₂, bicarbonate concentration, and base excess were monitored continuously with a multiparameter Paratrend (Diametrics Medical Inc, St Paul, Minn) sensor and compared between 0.3 and 0.5 gestation. Results: During maternal normoxemia all parameters were constant. At all gestational ages maternal hyperoxemia caused no changes apart from a rapid increase in amniotic fluid PO₂ (P <.001). Maternal hypoxemia led to a reduced amniotic fluid PO₂ (P <.001), whereas the PCO₂ and the bicarbonate concentration increased (P <.001). Changes in amniotic fluid gas values and acid-base balance were more pronounced at 0.3 gestation than at 0.5 gestation. Amniotic fluid Po₂ responded earlier to maternal hyperoxemia than to hypoxemia (9.5 vs 14.3 minutes; P <.001). During hypoxemia maternal PaCO₂ changed faster than did amniotic fluid PCO₂ (P <.001). Conclusions: Acute maternal hypoxemia during early pregnancy was quickly reflected in amniotic fluid gas values and acid-base balance, whereas hyperoxygenation induced the quickest changes in amniotic fluid PO₂. The metabolic effects of maternal hyperoxemia and hypoxemia were also more pronounced during early pregnancy than at mid pregnancy. (Am J Obstet Gynecol 2000;182:661-5.)