Doppler examinations of different uteroplacental vessels (uterine arteries, arcuate arteries), umbilical artery, fetal thoracic aorta, and median cerebral artery were performed on 55 patients with idiopathic preterm labor (24.5 to 32.5 weeks). Thirty normal pregnancies of corresponding gestational age served as a control group. Significant differences of median values between the preterm labor and control group were found only for the resistance index (RI) in the central arcuate artery and for the pulsatility index (PI) in the fetal thoracic aorta. In about twenty percent of pregnancies in preterm labor, pathological values of RI and PI in uteroplacental and fetal vessels account for the presence of an impaired perfusion. Elevated PI in the uterine artery placental site and normal RI in the fetal thoracic aorta, correlate significantly to a shorter prolongation of pregnancy, lower gestational age on birth, and lower birth weight. The combination of these two blood flow indices (maternal PI greater than 0.90 and fetal RI less than 0.90) allow us to predict a preterm birth in a high percentage of cases (sensitivity 87.5%, specificity 100%, positive predictive value 100%, negative predictive value 93%).