Variation in the T/QRS ratio of fetal electrocardiograms recorded during labour in normal subjects.

The T/QRS ratio of the fetal ECG was obtained during labour from 25 women with normal pregnancies. The poor signal-to-noise ratio of the unprocessed signals, chiefly due to baseline wander, led to a wide variation between individual measurements. This problem was overcome by data averaging, the ratio being expressed as a mean over 1-min epochs. The average T/QRS ratio of each labour record ranged from 4% to 23% with a mean of 10% (for all 25). The average range (between 5th and 95th centile) of the 1-min T/QRS ratios was 13% and there were no significant changes as labour progressed. The effect of contractions on the T/QRS ratio was measured from eight subjects and found to be inconsistent.