Interpretation of the fetal ECG during labor: the effect of uterine contractions.

This study was performed in order to investigate the fetal electrocardiogram (FECG) during uterine contractions associated with normal labor. Twenty-five patients with low risk pregnancy between 38-41 weeks gestation were studied during the active stage of labor. Both FECG and intra-uterine pressure are obtained in a conventional manner and are continually sampled into the computer. The FECG is averaged point-to-point, synchronized to the peak of the R-wave. This is performed by a QRS detection algorithm which is based on a digital analysis of slope, amplitude and width. A digital band-pass filter composed of cascaded high-pass and low-pass filters reduces false detections and permits the use of auto-adjustable low thresholds. A separate averaging is performed on the T-wave in order to prevent attenuation due to variable R-T interval. The T wave is subsequently aligned in time and position to the rest of the QRS complex. A significant increase was observed in the T/QRS amplitude ratio during the first half of the uterine contraction. Such an increase was also observed in the short and long-term FHR variability. No significant changes were observed in the other components of the FECG. In conclusion, by implementing a computer based system it is possible to analyse the FECG during labor. Based on this and previous studies it may well prove to be a sensitive indicator of fetal condition.