A continuous-wave Doppler unit was used to obtain umbilical artery velocity waveforms and to calculate the peak-systolic/diastolic ratio, a reflection of vascular resistance distal to the point of measurement. A total of 587 examinations were performed on 189 women between 18 to 42 weeks' gestation. The neonates were divided in four groups based on their centile birth weight: less than 25%, 25% to 50%, 51% to 75%, and greater than 75% for that gestational age. In all four groups the placental vascular resistance as reflected by the systolic/diastolic ratio fell progressively from the 24 weeks onward. In the lower weight group, placental vascular resistance between 31 to 39 weeks' gestation was higher than in the other three groups (p less than 0.05). For measurements taken between 31 and 39 weeks' gestation we calculated the predictive value of this test in identifying the small-for-gestational age fetus. A ratio of greater than or equal to 3 was defined as abnormal. For the entire population tested, sensitivity was 78% and the specificity was 83%. The predictive value of a positive test was 49%, but when hypertension was present, it was 66%. These findings may prove useful in identifying and managing pregnancies at risk for intrauterine growth retardation.