How blood viscosity influences changes in circulation during pregnancy?

The changes of whole blood viscosity and plasma viscosity were studied in 15 normal non-pregnant women, and 120 normal pregnant women ranging from 8 to 39 weeks of gestation. The values of whole blood viscosity during normal pregnancy were significantly lower from 20 to 31 weeks of gestation than those in the other periods of gestation, and there was a positive correlation between whole blood viscosity and hematocrit. Plasma viscosity, however, did not change significantly during pregnancy. There was no correlation between whole blood viscosity and plasma viscosity, and there was also no correlation between plasma viscosity and plasma fibrinogen concentration. These findings suggest that decreases in whole blood viscosity, resulting from the change of packed-cell volume, may strongly contribute hemorheologically to the decrease in peripheral resistance during the second trimester.