Doppler umbilical artery waveform indices--normal values from fourteen to forty-two weeks

Normal values for Doppler waveform indices of the umbilical artery have been reported for gestational ages of 20 to 40 weeks in small numbers of normal patients. We evaluated 590 patients studies performed at 2-week intervals from 14 to 42 weeks' gestation on patients without medical or pregnancy complications. Readings were obtained during fetal quiet times (no fetal breathing or movements). Values for A (systolic) and B (diastolic) pressures were plotted as Pourcelot (A - B/A) and A/B ratios. Mean, SD, and 95% confidence limits were derived, and the skewness, kurtosis, and regression correlations were calculated. No diastolic flow was found in any pregnancy greater than 15 weeks' gestation (n = 25) or in 50% of the gestations between 15 to 17 weeks (n = 25). When diastolic pressure equals zero, the Pourcelot ratio value equals one and the A/B ratio approaches infinity and loses meaning. Recent work by Thompson et al. suggests that the Pourcelot ratio fits a normal distribution from 20 to 40 weeks' gestation and that the A/B ratio (which does not fit a normal curve) may be transformed to a normal distribution by conversion of the A/B ratio to 1/1 - Pourcelot ratio. Our data supports the normality of both indices from 18 to 42 weeks' gestation, but these assumptions are not applicable as the Pourcelot ratio approaches one or as the A/B ratio approaches infinity. Knowledge of normal umbilical flow ratios at gestational ages from 18 weeks may allow early detection and directed management of high-risk pregnancies.