Intraobserver variation in Doppler ultrasound indices of placental perfusion derived from different numbers of waveforms.

Continuous-wave ultrasound is used to obtain records of Doppler-shifted frequencies from arteries. Indices using the maximum frequency envelope are usually derived from a number of selected waveforms on each record and averaged. Analysis of variance was performed on indices obtained from repeated Doppler ultrasound waveform records of uterine and umbilical perfusion in late pregnancy. Intraobserver variation was minimal when derived from six (consecutive) waveforms and was less than 10% for each index.