Intrapartum asphyxia is responsible for only a small proportion of cerebral palsy cases, although obstetricians are often held accountable. Umbilical cord pH and blood gas values provide valuable information regarding the status of the infant at birth; base excess determination quantifies the magnitude of metabolic acidosis, the putative risk factor for central neurologic injury. Human and animals studies have confirmed normal values of base excess before labor, and consistent rates of base excess change in relation to the degree of fetal hypoxemia or heart rate patterns. Thus, the combination of assumed base excess values before labor and measured values after birth, together with an assessment of degrees of fetal hypoxemia during labor, permits an interpolation of fetal base excess values throughout the course of labor. Because threshold levels of base excess (eg, -12 mmol/L) have been associated with an increased risk of neonatal neurologic injury, this approach provides a framework for the assessment of fetal heart rate tracings during labor and, potentially, the timing of hypoxic/ischemic injury.