Blood viscosity studies were carried out at regular intervals in 51 patients with normal pregnancy and in 27 patients with high risk pregnancy, i.e. with EPH-gestosis and placental insufficiency. During gestation the relative blood viscosity is significantly increased compared to 15 normal non-pregnant women. At a corrected hematocrit of 45% we found a direct correlation to plasma fibrinogen and to the blood sedimentation rate. Although the whole blood viscosity does not indicate any change in normal pregnant women, there is, however, a viscosity increase in the last trimester of high risk pregnancies. Additionally a mathematical model of the oxygen diffusion in the placental cotyledo demonstrates rapid decrease of the oxygen partial pressure in the maternal intervillous channel if the microcirculation of the "Placenton" is impaired.