Abstract

Blood viscosity was measured in 14 healthy, menstruating women, aged 17-51 years and in 10 healthy, postmenopausal women, aged 55-64 years. The fertile women were studied once a week during a normal menstrual cycle and the postmenopausal women twice with an interval of 2 weeks.  Blood viscosity was measured at natural hematocrit as well as at hematocrit 45%. In the postmenopausal women no changes in blood viscosity were found.  In the fertile women, blood viscosity at hematocrit 45% was lowest at the start of the menstrual bleeding and increased to a peak at day 7 (p < 0.01), with a similar pattern when measured at natural hematocrit.  Plasma viscosity also had its lowest value at the onset of menstrual bleeding, increasing to a maximum at day 21. Changes in plasma triglycerides, but not in fibrinogen or cholesterol, seemed to contribute to this increase. Plasma factors only partly explained the variations in blood viscosity, and changes in red cell properties were also found to be of importance.  The clinical significance of these rheological changes remains to be established, but at least theoretically there may be an increased risk for thromboembolism, e.g. at surgery, during days 5-15 of the cycle.  In studies on blood flow and rheological conditions in fertile women, it seems advisable to standardize for time in the menstrual cycle.