Between 13 and 40 weeks gestation, we recorded Doppler signals from the umbilical artery of 25 women with normal pregnancy. Power spectral density graphics were derived from umbilical artery Doppler signals using Short Time Fowler Transformation and Autoregressive Modeling. Scattered values and regression curves for f(rang) values derived from power spectral density graphics were obtained In the STFT and AR power spectral density graphics of pregnant subject; frequency range (f(range)) of power spectral density graphics was lower at the beginning of gestation and increased progressively until term showing a pattern of high frequency range. Analysis of variance showed statistical significance (for AR method p =0.0004 for STFT method p =0.0009) for all regressions performed.