The fetal circulation has been an exciting area of study for centuries. The principles which grew from the period of hypotheses have been demonstrated in several animal models. These experiments have shaped the major concept of fetal circulation. More recently, the improvement in ultrasound technology has allowed a non invasive study of the fetal circulation in humans. Although the general schema of the fetal circulation has been confirmed in humans, in some aspects some substantial differences have been demonstrated. They may not only reflect some inter-species differences, but also underscore the limitation of chronically instrumented animal studies.