Abstract
1) To describe a new method for measuring uterine vascular resistance (Ruta) noninvasively and establish longitudinal reference ranges for the uterine artery (UtA) blood flow velocities, Doppler indices and Ruta at 22 + 0–39 + 6 weeks of gestation. 2) To explore the relationship between UtA Doppler indices and Ruta during the second half of pregnancy. This was a longitudinal observational study of 53 low-risk pregnancies (253 observations). Doppler ultrasonography was used to measure UtA velocities and the pulsatility index (PI), resistance index (RI) and systolic/diastolic (S/D) ratio were calculated. Maternal blood pressure was measured and the Ruta was calculated as a ratio between the mean arterial pressure and UtA time-averaged maximum velocity. At 22 + 0–39 + 6 weeks of gestation, the mean Ruta decreased from 1628 to 1180 dyne.s/cm3 (P < 0.0001). The UtA absolute velocities increased steadily throughout the second half of gestation (P < 0.0001), whereas the UtA PI (0.79 to 0.56), RI (0.51 to 0.40) and S/D ratio (2.0 to 1.7) decreased (P < 0.0001). Significant positive associations were found between the Ruta and the UtA Doppler indices (P = 0.003–0.001). We have described a simple non-invasive method for measuring Ruta based on blood flow velocity rather than volume blood flow and provide reference ranges for serial evaluation of UtA absolute velocities, Doppler indices and Ruta in the second half of pregnancy.
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