Hematological adaptation

Hematological adaptation to pregnancy is summarized in T§.b,!§.2..

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Table 2 Hematological adaptation to pregnancy

The plasma volume increases progressively throughout normal pregnancy. Most of the 50 per cent increase occurs by 34 weeks and is positively correlated with the birth weight of the baby. Because the expansion in plasma volume is greater than the increase in red cell mass, there is a fall in hemoglobin concentration, hematocrit, and red cell count. Despite this hemodilution, there is usually no change in mean corpuscular volume or mean corpuscular hemoglobin concentration.

The platelet count tends to fall progressively during normal pregnancy, although it usually remains within normal limits. In some women, the count will reach thrombocytopenic levels by term. Burrows...a.D.d KejiOD.i1990). found that 7.6 per cent of 6715 pregnant women had thrombocytopenia ((100-150)^10 9/l) at term, but at least three-quarters of these women had 'pregnancy-associated thrombocytopenia'. This benign condition requires no intervention. Differentiation between idiopathic thrombocytopenic purpura and 'essential thrombocytopenia of pregnancy' is possible in specialist laboratories with antiplatelet antibody determination, but the assay is not readily available. Thrombocytopenia documented in the first half of pregnancy is less likely to be due to the pregnancy itself, and should alert the clinician to a possible diagnosis of idiopathic thrombocytopenic purpura.

Pregnancy causes a two- to threefold increase in the requirement for iron, and a 10- to 20-fold increase in folate requirements. These increased demands are often not met by diet alone, a fact which provides the rationale for routine supplementation with oral hematinics. All women planning a pregnancy are now advised to take 0.4 mg folate daily periconceptually as a prophylactic agent against neural tube defects and other fetal abnormalities. In addition, women who have had a previous fetus with a neural tube defect and women taking anticonvulsants are advised to take 5 mg folate daily periconceptually.

Changes in the coagulation system during pregnancy produce a physiological hypercoagulable state (presumably to stop bleeding following delivery). The concentrations of certain clotting factors, particularly factors VIII, IX, and X, are increased. Fibrinogen levels rise significantly by up to 50 per cent. Fibrinolytic activity is decreased and the concentration of endogenous anticoagulants such as protein S falls. Thus pregnancy alters the balance within the coagulation system in favor of clotting, predisposing the pregnant and postpartum woman to venous thrombosis. This additional risk is present for at least 6 weeks following delivery. The in vitro tests of clotting remain normal in the absence of anticoagulants or a coagulopathy.

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