The increasing interest in arterial duct flow patterns in the fetus warrants the establishment of an accurate range of normal flow parameters throughout gestation. We therefore undertook a prospective echocardiographic study of 181 normal fetuses from the 16th to the 40th week of gestation. Adequate Doppler interrogation of the duct was obtained in 71% of the fetuses examined. Peak gradient, mean gradient and flow velocity integral in systole and diastole were digitized. The peak systolic gradient throughout pregnancy measured 2.7 +/- 1.4 mmHg with a slight tendency to increase with gestational age (r = 0.58). The peak ratio, defined as peak systolic gradient divided by peak diastolic gradient (28.1 +/- 14.9), did not vary significantly with gestational age. This time-independent index complements peak systolic flow in the assessment of normal and abnormal ductal flow. The definition of the normal range for ductal flow parameters, based on this relatively large fetal population, should facilitate the accurate diagnosis of fetal duct constriction.