9.1: Normal haematological values in pregnancy
During normal pregnancy, physiological changes in the mother affect the reference range for haematological parameters. Knowledge of these changes helps to avoid unnecessary blood transfusions caused by misinterpretation of blood count results:
- Maternal plasma volume increases by around 50% above the non-pregnant value by the late second trimester. Red cell mass only increases by 25–30%, resulting in a fall in Hb concentration (‘physiological anaemia of pregnancy’).
- Up to 10% of healthy pregnant women have a count below the non-pregnant reference range of 150–400×109/L at term (‘gestational thrombocytopenia’). The count rarely falls below 100×109/L and there is no increase in bleeding risk.
- Many coagulation factors, including plasma fibrinogen and Factor VIIIc, are increased in normal pregnancy and the anticoagulant factor Protein S is reduced. This contributes to the increased risk of thrombotic complications in pregnancy.