Influence of fetal breathing and movements on variability of umbilical Doppler indices using different numbers of waveforms.

Umbilical artery continuous-wave Doppler ultrasound waveforms were obtained from three groups of fetuses in late gestation: during quiescence, fetal breathing movements, and fetal body movements. The coefficients of variation, calculated from three values for each Doppler index, which were derived from one, two, four, six and eight waveforms, decreased as the number of waveforms increased. However, statistical analysis showed no significant improvement in variance when more than four waveforms were used during fetal quiescence and breathing and when more than six waveforms were used during fetal movement. A minimum of six waveforms ensured coefficients of variation less than or equal to 10% even during fetal breathing and fetal movements.