Reference values for blood flow velocity in the uterine artery in normal pregnancies from 18 weeks to 42 weeks of gestation calculated by automatic Doppler waveform analysis.

ABSTRACT

PURPOSE: The goal of the present study was to establish new Doppler reference ranges for maternal heart rate, intensity-weighted mean blood flow velocities (Vmean) and impedance indices (PI, RI) for the uterine artery by automated waveform analysis. MATERIALS AND METHODS: A cross-sectional prospective study of 921 low-risk pregnancies was performed at 18 - 42 weeks of gestation. Uterine blood flow velocities were derived with pulsed-wave color Doppler. Measurements were carried out 1 to 2 cm above the crossing of the uterine and external iliac arteries. Reference ranges for the individual measuring parameters were constructed based on a growth function from a four-parameter class of monotonic continuous functions according to the smallest square principle. RESULTS: A significant increase in intensity-weighted mean uterine blood flow velocities was observed at 18 - 42 weeks of gestation (Vmean = 43 cm/s to 50 cm/s (p < 0.001)). Reference curves for the pulsatility and resistance indices (PI, RI) significantly decreased with progressing gestation (PI 18 weeks 0.89; 42 weeks 0.65 and RI 18 weeks 0.45; 42 weeks 0.35). No significant PI and RI differences were observed when different placental locations were compared. The maternal heart rate decreased from 88 bpm to 77 bpm. CONCLUSION: Normal ranges for blood flow velocities and impedance indices in the uterine artery were established by Doppler ultrasound antenatal examinations of a large population of low-risk pregnancies. The data are proposed as reference curves to allow the early diagnosis of maternal and fetal risks.