Background: Variations in maternal blood parameters, which are mostly induced by the physiological changes that occur during pregnancy, have been reported in different gestational periods. The use of the established reference intervals for healthy adult females leads to the misclassification of healthy pregnant women as abnormal. Our aim was to establish appropriate reference intervals for biochemical, haematological and haemostatic parameters in the first and third trimesters of pregnancy. Methods: We included 565 healthy pregnant women with normal pregnancies. Blood samples were collected for biochemical analyses, complete blood counts and coagulation analyses at 8–12 and 28–37 weeks of gestation. The median and reference intervals (the 2.5th and 97.5th values) were calculated for each parameter during pregnancy and then compared to the established reference intervals for healthy adult females. Results: Significant increases in triglyceride, total cholesterol, low-density lipoprotein cholesterol, uric acid, alkaline phosphatase, white blood cell, mean platelet volume, fibrinogen and D-dimer reference intervals and clear decreases in total protein, albumin, blood urea nitrogen, creatinine, red blood cell, haemoglobin, haematocrit, platelet counts and thrombin time reference intervals were observed during pregnancy. According to the ‘n%’, most changes were observed beginning in the first trimester. Compared to the established reference intervals, the greatest misclassifications were observed for ALB, ALP and D-Di. Conclusions: Changes in maternal blood parameters during pregnancy were confirmed. We recommend that the reference intervals for most blood parameters be revised to account for the gestational period.