The advent of sonography has changed the practice of obstetrics by providing a window to the womb through which the anatomic structure of the fetus can be evaluated. The addition of Doppler flow studies of maternal and fetal vessels has provided a tool where the physiology of the maternal-fetal unit can be assessed. This information can provide the physician and the patient with vital information for a subsequent approach to the pregnancy. The use of fetal Doppler blood flow studies has become common in the evaluation and management of pregnancies complicated by conditions such as suspected fetal growth restriction and red blood cell isoimmunization to guide intrauterine therapy and delivery. The most commonly assessed Doppler flow studies of the fetus are the umbilical artery and middle cerebral artery (MCA). Doppler flow studies of the MCA are used in the assessment of the fetus at risk for anemia and growth-restricted fetus. Doppler flow studies of the umbilical artery can reflect abnormalities in "down-stream" or the fetal side of placental resistance, and the assessment of the maternal vasculature evaluates "up-stream" blood flow or the maternal side of placental resistance. The purpose of this review is to describe the clinical utility of uterine artery Doppler flow studies in the prediction of adverse pregnancy outcomes in low and high risk populations.