In the past 20 years, measurements of umbilical blood flow and umbilical venous Po₂, oxygen saturation, pH, and oxygen capacity have provided reliable information about the state of oxygenation of normal and growth restricted human fetuses. However, no comparable information is available about the uterine circulation. Therefore, understanding of oxygen transport across the human placenta and the effect of maternal ventilation on fetal oxygenation is tentative, and currently based on a model that is derived from evidence in another species. The main purpose of this model is to illustrate the kind of information that is needed to make further progress in this area.