Fetal heart tracings (FHTs) are useful as a window into the oxygenation status of the fetal brain. Patterns in the FHT reflect the oxygen status of the fetal brain. Fetal adaptive response to progressive hypoxemia and acidosis are detectable and produce recognizable patterns in the fetal heart rate. The basic physiology and adaptive responses that regulate the fetal heart rate and physiological fetal adaptations to stress as reflected in the FHTs are described. Mechanisms of oxygen delivery to the fetus including ways in which those mechanisms can be disrupted are reviewed.