INTRODUCTION: Recent studies have shown increased RBC aggregation and no difference in plasma viscosity in the presence of markedly lower hematocrit in women at term compared to non-pregnant women. Little is known about the outcome of blood rheological parameters and red blood cell (RBC) deformability particularly in the course of normal pregnancy. METHODS: During a 36 months interval 1.913 blood samples were randomly collected from a total of 945 pregnant women in the course of their pregnancy (n = 1.259) and during puerperium (upto 1 week; n = 654). Next to the blood count, hemorheological parameters including red blood cell (RBC) -aggregation (stasis E0; low shear E1), -deformability (low, moderate and high shear conditions) and plasma viscosity (pv) were assessed. Plasma viscosity (pv) was examined using KSPV 1 Fresenius, RBC aggregation (stasis: E0 and low shear: E1) using MA1-Aggregometer; Myrenne and RBC deformability (def) was determined by Rheodyn SSD Diffractometer, Myrenne, Roetgen, Germany were tested. In some of these women laboratory results prior to pregnancy (n = 145) were available which were compared with those during pregnancy. RESULTS: Mean maternal pv remained unchanged within each trimester and compared to the values before pregnancy and during early puerperium (Range of means: 1.18–1.20 mPa S). In contrast, RBC agg (E0 and E1) was markedly higher in the 2nd (21.8 ± 7.0 and 28.9 ± 9.4; p < 0.001) and 3rd trimester (18.74 ± 8.4 and 28.2 ± 9.4; p < 0.01) compared to the values before pregnancy (16.4 ± 6.4 and 20 ± 7.5) and during 1st trimester (17.49 ± 6.5 and 22.4 ± 7.4). There was a stat. significant temporary reduction in RBC def. under all shear rate conditions during 2nd trimester compared to the values before pregnancy which remained significantly lower during 3rd trimester only under high shear rates. An increase RBC agg was stat. significantly inversely correlated with reduced RBC def being most pronounced under low shear rate conditions. While RBC rigidity was stat. significantly correlated with higher hematocrit values there was only a weak correlation between RBC agg and haematocrit (E0: r = –0.084; p = 0.03; E1: r = –0.06; p = 0.1). Pv was not correlated with haematocrit or RBC def but stat. significantly correlated with RBC agg. CONCLUSIONS: Blood rheological changes manifest during 1st trimester, and fairly remain unchanged during 2nd trimester until term. Physiologic hemodilution and increasing hypercoagulability is accompanied by high RBC -aggregation and – rigidity during 2nd trimester while plasma viscosity remains nearly unaffected throughout normal pregnancy.