OBJECTIVE: The addition of ST waveform analysis (STAN, Neoventa, Sweden) to fetal heart rate (FHR) tracings has been demonstrated to improve fetal outcome and reduce operative delivery rates, though the actual level of fetal acidosis at which STAN indicates intervention has not been assessed. We sought to determine if FHR ST segment analysis recommends intervention at appropriate levels of fetal acidosis. METHODS: FHR tracings of 10 acidotic and 10 non-acidotic infants with FHR tracings having a minimum of one STAN flag were retrospectively analyzed. Fetal base deficit (BD) was calculated by interpolation throughout the FHR tracing and STAN 'action' and 'ignore' flags assigned a fetal BD value. A secondary analysis was performed with a revised interpretation of FHR reassuring status. RESULTS: The mean (+/-SD) BD of the first STAN action was significantly greater than the first 'ignore' (4.0+/-2.1 vs. 3.0+/-0.8 mmol/L, p