OBJECTIVE The purpose of this study was to examine the longitudinal relationship between Doppler flow velocity waveforms of the spiral artery (SA) and uterine artery (UA) in pregnant women. METHODS Ninety-seven primigravidas with uncomplicated singleton pregnancies were analyzed. Spiral artery and combined UA velocity waveforms were assessed by transabdominal color Doppler sonography at gestational ages of 11 through 13, 14 through 17, and 18 through 24 weeks; each measurement was performed twice. The pulsatility index (PI) was calculated for the left and right UA, and the results were averaged as a combined UA. In addition, the presence of UA bilateral notching was reported. Bland-Altman plots and generalized estimating equations were used to assess intraobserver variability and the longitudinal relationship between SA and UA blood velocities. RESULTS A total of 284 UA and 263 SA Doppler flow measurements were analyzed. Intraobserver variability rates for the SA and UA were 0.54 and 0.90, respectively. Results showed a continuous decrease of the mean PI in the SA and UA with increasing gestational age. Uterine artery bilateral notching was reported in 35%, 9%, and 3% of the cases at gestational ages of 11 through 13, 14 through 17, and 18 through 24 weeks. Generalized estimating equation analysis showed a significant correlation (r = 0.41) between the SA and UA (P < .0001). CONCLUSIONS Uterine artery Doppler measurements in early pregnancy seem to accurately reflect peripheral resistance of SAs. Furthermore, trophoblastic invasion seems a continuous process in the first half of pregnancy, in which early UA bilateral notching is a physiologic event.