Oscillation-frequency and baseline fetal heart rate in the last 30 minutes of labour.

The last 120 minutes of 394 intrapartum fetal heart rate (FHR) records of high technical quality were evaluated. Baseline FHR was estimated and oscillation frequency (OF) counted for each minute. There was a highly significant association between the baseline FHR and the number of oscillations/minute; an increase in baseline FHR of 10 beats/minute was associated with a fall of about one oscillation/minute. The highest mean OF (9.6 +/- 4.8 events/minute) was found in fetuses with an umbilical artery pH of between 7.20 and 7.25. The baseline FHR should be taken into account when assessing fetal condition.