[Physiologic basis for fetal heart rate analysis during labour]

Fetal monitoring during labour aims to identify fetal acidosis responsible for brain lesions or perpartum death. In France, continuous recording of fetal heart rate (FHR) is largely used during labour. However, FHR interpretation remains difficult, as no specific sign for fetal hypoxia or acidosis have been described. Fetal heart rate is regulated by the autonomous nervous system including the chemoreflex, the baroreflex and the central nervous system. Appropriate regulation is conditioned by tissue oxygenation. Therefore, basal FHR physiologic instability is reduced when fetal hypoxia or acidosis occur, leading to reduce variability. FHR decelerations are frequent during labour due to either fetal response to physiologic modification of oxygenation during labour or fetal hypoxia, which should be differentiated in order to avoid inappropriate intervention. Knowledge of physiologic mechanisms involved in FHR regulation and fetal haemodynamic adaptation to hypoxia could help in learning FHR interpretation. This review is exposing the factors regulating FHR and the mechanisms involved in fetal circulatory responses to hypoxia and acidosis.