Uteroplacental Doppler Flow Velocity Waveform Indices in Normal Pregnancy: A Statistical Exercise and the Development of Appropriate Reference Values

ABSTRACT

In a prospective cross-sectional study, we examined 154 normal pregnant women and measured the systolic to diastolic (S/D) ratio and resistance index (Rl) in the umbilical artery and both uterine arteries. Placental location with respect to laterality was determined by real-time ultrasound. In patients with unilateral placental location, each uterine artery was evaluated according to its relationship with the placenta. Doppler flow velocity waveforms were obtained by a continuous wave Doppler device. Kolmogorov D tests revealed that Rl values follow gaussian distribution, but that S/D values were markedly skewed to the right. There was a significant negative linear relationship between gestational age and umbilical artery Rl and a significant negative curvilinear relationship between gestational age and umbilical artery (S/D (r = 0.83, p <0.001; and r = -0.79, p <0.001, respectively). Confidence bands for umbilical artery Rl were developed based on the linear model with gestational age (fitted umbilical artery Rl = 0.97199 - 0.01045*gestational age). Confidence bands for umbilical artery S/D were derived from the corresponding Rl values by means of the functional relationship S/D = 1/(1 - Rl). The Rl and S/D values of the uterine arteries declined until 24 to 25 weeks' gestation and remained unchanged thereafter. This relationship, however, was not statistically significant (r = -0.10, p = 0.22). The placental uterine artery is different from the nonplacental quantitatively and qualitatively. We suggest that properly derived reference values should be used when Doppler flow velocity waveform analysis is utilized in the management of high-risk pregnancies.