Cardiotocography (CTG) is a technique used to monitor intrapartum fetal condition and is one of the most common obstetric procedures. Second line methods of fetal monitoring have been developed in an attempt to reduce unnecessary interventions due to continuous cardiotocography and to better identify fetuses at risk of intrapartum asphyxia. The acid-base balance of the fetus is evaluated by fetal blood scalp samples, the modification of the myocardial oxygenation by the fetal ECG ST-segment analysis (STAN) and the autonomic nervous system by the power spectral analysis of the fetal heart variability. To correctly interpret the features observed on CTG traces or second line methods, it seems important to understand normal physiology during labor and the compensatory mechanisms of the fetus in case of hypoxemia. Therefore, the aim of this review is first to describe fetal physiology during labor and then to explain the modification of the second line monitoring during labor.