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Notes: Summary. Blood viscosity and several of its determinants (packed cell volume, plasma viscosity, plasma fibrinogen and erythrocyte deformability) and several haemostatic variables (platelet count, serum fibrin degradation products and plasma soluble fibrin) were measured in 106 women during normal third-trimester pregnancy, 12 patients with moderate pre-eclampsia, nine patients with severe pre-eclampsia and 16 patients with confirmed fetal growth retardation. Blood viscosity was measured at high and low shear rates (94.5 and 0.94 s−1), with and without correction to a standard packed cell volume of 0.45. In normal pregnancy low-shear blood viscosity increased significantly towards term, associated with significant increases in packed cell volume and plasma viscosity (but not fibrinogen); there was no change in high-shear blood viscosity at native packed cell volume and a decrease in viscosity after correction for packed cell volume, associated with a significant increase in erythrocyte deformability. Significant decrease in the platelet count and significant increases in serum fibrin degradation products and plasma soluble fibrin suggested a degree of haemostatic activation in normal third-trimester pregnancy. In moderate pre-eclampsia blood viscosity was significantly lower at low shear rate, due to a significantly lower packed cell volume and despite a significantly elevated plasma viscosity. In severe pre-eclampsia high-shear blood viscosity was significantly elevated after correction for packed cell volume, associated with a significantly reduced erythrocyte deformability; plasma fibrinogen levels were reduced, but other variables were not significantly abnormal. In fetal growth retardation blood viscosity was significantly increased at both shear rates, despite a reduced packed cell volume: this was also associated with a significant reduction in erythrocyte deformability. In general there was little correlation between blood viscosity factors and haemostatic variables, but in normal-term pregnancy soluble fibrin levels correlated with blood viscosity at both shear rates. We suggest that increased blood viscosity and decreased erythrocyte deformability may play a role in the placental insufficiency of severe pre-eclampsia and fetal growth retardation.