The relationship between peak-systolic/end-diastolic ratio of the umbilical artery waveform and fetal biometry was studied in 127 uncomplicated pregnancies with established dates between 20 and 40 weeks' gestation. At each ultrasonographic examination fetal biometry included measurement of the biparietal diameter, head circumference, abdominal circumference, and femur length. The peak-systolic/end-diastolic ratio was measured by either a continuous or a pulsed-wave method. There were significant linear negative correlations between all the biometric parameters, as well as between the ultrasonographically estimated fetal weight and peak-systolic/end-diastolic ratio. Of the individual ultrasonographic parameters the femur length (for gestations less than 30 weeks) and the abdominal circumference (for gestations greater than or equal to 30 weeks) were found to be best correlated with the peak-systolic/end-diastolic ratio. Regression curves, including the 10th and the 90th percentile, were developed between each biometric parameter (biparietal diameter, head circumference, abdominal circumference, and femur length), as well as between estimated fetal weight and peak-systolic/end-diastolic ratio. The estimated fetal weight nomogram had the best sensitivity (48%) in predicting intrauterine growth retardation. These nomograms should prove most useful in assessing downstream placental vascular resistance in high-risk patients with unknown dates.