We performed power spectral analysis in the 10 to 12 week human fetus during conditions of maternal spontaneous and stop breathing to explore whether umbilical resistive and capacitive impedance to flow components vary synchronously, as seen in the pre-innervated chick circulation. Recordings were made from the floating loop of the umbilical artery using a Toshiba SHH140A pulsed-Doppler velocimeter with 6 MHz transvaginal probe. Ten velocity time series were recorded during spontaneous breathing from a total eight fetusses at weeks 10 (n=2), 11 (n=3), and 12 (n=3). Ten non-matched velocity time series were recorded during stop-breathing from eight fetuses at weeks 10(n=5), 11 (n=1), and 12 (n=2). Ultrasound spatial peak temporal average was less than 100mW/cm^2. Doppler forward flow audio signals were recorded at 44 kHz for intervals greater than 16 seconds. Peak-velocity, meanvelocity, and instantanious heart rate (beat-to-beat) in each cardiac cycle were determined in each velocity trace, along with the time-series-mean heart rate (MHR) and time-series-mean-velocity (MV). Peak-velocity variability (PVV) and mean-velocity variability (MVV) were determined, de-trended via low-order polynomial fit to remove DC-drift, and power spectral densities were computed using FFT. To compensate for MHR differences between fetus, we normalized the spectral frequency axes by the respective MHR. For comparison purposes, the normalized frequency axes ranged from zero (DC) to 1 (heart rate). To compensate for MV differences between fetus, we computed the ratio of spectral power in 10 pre-set bands (0.1 increments of normalized frequency) to total spectral power. The average power ratios for PVV during stop breathing were 27%, 26%, 19%, 15%, 7%, 3%, 1%, <1%, <1%, and <1%, respectively, in each band. During spontaneous maternal breathing these ratios changed to 23%, 40%, 14%, 10%, 6%, 3%, 1%, <1%, <1%, and <1%, respectively. The average power ratios for MVV during stop breathing were 32%, 27%, 20%, 13%, 5%, 2%, 1%, <1%, <1%, <1%, and <1%, respectively. During spontaneous maternal breathing these ratios changed to 32%, 37%, 13%, 8%, 5%, 2%, <1%, <1%, <1%, and <1%, respectively. While the absolute spectral power for PVV is some 15-times greater than that for MVV, normalized spectral comparison suggests synchronous variability of umbilical resistive and capacitive impedance components.