BACKGROUND: The diagnostic value of ST analysis of the fetal electrocardiogram (fECG) during labor is uncertain. False alarms (ST events) may be explained by physiological variation of the fetal electrical heart axis. Adjusted ST events, based on a relative rather than an absolute rise from baseline, correct for this variation and may improve the diagnostic accuracy of ST analysis. AIMS: Determine the optimal cut-off for relative ST events in fECG to detect fetal metabolic acidosis. STUDY DESIGN: Post-hoc analysis on fECG tracings from the Dutch STAN trial (STAN+CTG branch). SUBJECTS: 1328 term singleton fetuses with scalp ECG tracing during labor, including 10 cases of metabolic acidosis. OUTCOME MEASURES: Cut-off value for relative ST events at the point closest to (0,1) in the receiver operating characteristic (ROC) curve with corresponding sensitivity and specificity. RESULTS: Relative baseline ST events had an optimal cut-off at an increment of 85% from baseline. Relative ST events had a sensitivity of 90% and specificity of 80%. CONCLUSIONS: Adjusting the current definition of ST events may improve ST analysis, making it independent of CTG interpretation.