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Haematological Changes During Pregnancy

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1. Blood Volume

• Total blood volume rises by ~45-50% in singleton pregnancies; up to 75-100% in twin/triplet pregnancies
• Expansion begins in the first trimester, accelerates rapidly in the second trimester, slows in the third trimester, and plateaus in the last few weeks
• The increase serves to:
  • Supply the hypertrophied uterine vasculature
  • Protect against impaired venous return (supine/erect positions)
  • Buffer against blood loss at delivery (300-500 mL vaginal; 800-1000 mL caesarean)

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2. Plasma Volume

• Progesterone and oestrogen cause peripheral vasodilation and decreased SVR, reducing renal perfusion
• The RAAS responds with increased aldosterone, augmenting sodium and water reabsorption
• Renal threshold for AVP shifts leftward (plasma osmolality set-point decreases), further expanding volume
• After placental delivery, oestrogen/progesterone fall abruptly and a vigorous post-partum diuresis follows
• Hypervolaemia also occurs with trophoblastic disease (a fetus is not essential for its development)
• Creasy & Resnik's Maternal-Fetal Medicine; Medical Physiology

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3. Red Cell Mass and Haemoglobin

• Because plasma volume rises more (~50%) than RBC mass (~25%), a dilutional or "physiologic" anaemia develops - this is normal and not pathological
• Total oxygen delivery is not reduced because cardiac output increases to compensate
• Iron supplementation corrects the RBC deficit; without it, haemoglobin can fall as low as 11 g/dL and haematocrit to 33%
• In twin/triplet pregnancies, the plasma expansion is even greater and anaemia more pronounced
• Miller's Anesthesia 10e; Creasy & Resnik's Maternal-Fetal Medicine

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4. Iron and Erythropoiesis

• Pregnancy substantially increases iron demand to support:
  • Expanded maternal RBC mass
  • Fetal and placental iron requirements
• Erythropoietin levels rise in pregnancy, stimulating erythropoiesis
• Iron deficiency anaemia is the most common cause of true anaemia in pregnancy (haemoglobin <11 g/dL)
• Without supplementation, iron stores are readily depleted, especially in the third trimester

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5. White Blood Cells (Leukocytes)

• Physiologic leukocytosis is common in pregnancy and is unrelated to infection
• Predominantly a neutrophilia
• WBC rises further during labour (proportional to the duration of labour)
• This physiologic leukocytosis can overlap with SIRS/sepsis criteria, making infection harder to diagnose in pregnancy
• Miller's Anesthesia 10e

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6. Platelets

• The mild decrease is due to haemodilution and more rapid platelet turnover
• Gestational thrombocytopenia is a diagnosis of exclusion - must differentiate from:
  • Idiopathic thrombocytopenic purpura (ITP)
  • HELLP syndrome (haemolysis, elevated liver enzymes, low platelets)
  • Pre-eclampsia
• Gestational thrombocytopenia is not associated with abnormal bleeding and resolves postpartum
• Miller's Anesthesia 10e

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7. Coagulation - Hypercoagulable State

Increased Factors (Pro-coagulant)

Decreased Factors (Anti-coagulant)

Net Effect

• PT shortened by ~20%
• PTT shortened by ~20%
• Overall: shortened clotting times, promoting haemostasis

Thromboelastography (TEG) at Term

• Decreased R time (time to start clot formation)
• Decreased K time (time to specified clot strength)
• Increased α angle (rate of clot formation)
• Increased MA (maximum amplitude / clot strength)
• Changes begin as early as the first trimester
• Miller's Anesthesia 10e; Harrison's Principles 22e

Clinical implication: This hypercoagulable state, combined with venous stasis and endothelial injury, constitutes Virchow's triad in pregnancy - the basis for a 4-6x increased risk of VTE (venous thromboembolism) during pregnancy and the puerperium.

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8. Fibrinogen

• Normal pregnancy fibrinogen: 400-450 mg/dL (significantly above the normal non-pregnant range of 200-400 mg/dL)
• Values below 300 mg/dL in pregnancy indicate significant consumption of coagulation factors (e.g. DIC, abruption, HELLP)
• Oestrogens stimulate hepatic rough endoplasmic reticulum, increasing protein synthesis including fibrinogen, factors VII, VIII, IX, and X
• Creasy & Resnik's Maternal-Fetal Medicine; Rosen's Emergency Medicine

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9. Summary Table

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10. Key Clinical Points

1. Physiologic anaemia is expected - Hb ~11.6 g/dL is normal at term; values below 11 g/dL need investigation
2. Leukocytosis up to 13,000/mm³ is normal - do not treat for infection without other evidence
3. Mild thrombocytopenia is usually gestational - but HELLP and ITP must be excluded
4. Pregnancy is a hypercoagulable state - elevated DVT/PE risk throughout pregnancy and 6 weeks postpartum
5. Post-partum autotransfusion - uterine contraction after delivery returns >500 mL blood to circulation, offsetting delivery blood loss
6. Iron supplementation is routinely indicated to support the expanded RBC mass and prevent true iron-deficiency anaemia