Fetal adaptive responses to asphyxia.

The fetal environment is thus well suited for normal growth and development with oxygen availability exceeding oxidative needs. With impairments in blood gas exchange this excess oxygen acts as a "margin of safety," providing for the maintenance of oxidative metabolism through increases in fractional O2 extraction, although with resultant fetal hypoxemia. Increases in blood O2 capacity and redistribution of cardiac output in response to this hypoxemia further protect fetal oxygenation. Additional adaptive mechanisms involve a decrease in energy-consuming processes, including growth restriction, decreasing fetal movements, and behavioral state alterations. Although protective in so far as essential metabolic functions are maintained, pathologic change may occur as the "oxygen margin of safety" becomes limited or energy-conserving measures give rise to abnormal growth and development.