Haematology and pregnancy

Published on 09/03/2015 by admin

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Last modified 09/03/2015

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Chapter 50 Haematology and pregnancy

Haematologic changes in pregnancy

Plasma volume increases during pregnancy by up to 50%, and is maximal at around 34 weeks gestation. Red blood cell mass increases by only 17%, resulting in a relative haemodilution. This leads to a reduction in haemaglobin, haematocrit and red blood cell count, without changing the mean corpuscular volume or mean corpuscular haemaglobin concentration.

Iron requirements increase throughout pregnancy, with 60 mg/day of elemental iron needed during the second and third trimesters. Folate requirements increase from 50μg/day in the non-pregnant woman to around 400 μg/day. Pregnancy has minimal impact on vitamin B₁₂ levels, as most adults have a 2–3-year store available.

Platelet count remains in the normal range for most pregnant women, though the average is slightly lower than in the non-pregnant population. The lower limit of normal is between 106 and 120 x10⁹/L. Pregnancy is associated with a leucocytosis due to an increase in circulating neutrophils. The average white blood cell count in the second and third trimesters is between 9 and 15 x10⁹/L.

Anaemia in pregnancy

Anaemia is a reduction in red cell mass relative to plasma volume, measured as the amount of haemaglobin (Hb) per litre of blood volume. Anaemia in pregnancy is defined as an Hb <110 g/L in the first and third trimesters, and <105 g/L in the second trimester. Around 20% of women are anaemic in the third trimester of pregnancy. Anaemia should be investigated and treated, as severe anaemia has been associated with miscarriage, preterm birth, intrauterine growth restriction (IUGR) and fetal death.

Causes of anaemia

• microcytic: iron deficiency, haemoglobinopathies, chronic disease

• normocytic: haemolysis, haemorrhage

• macrocytic: vitamin B₁₂ or folate deficiency

Iron deficiency

• Iron requirements average around 1000 mg over the course of a normal pregnancy; 300 mg is required for the fetus and placenta, 500 mg for the expansion of maternal Hb, and 200 mg is lost through the gut, urine and skin.

• Iron deficiency is responsible for 75% of cases of anaemia in pregnancy.

• Prevalence may approach 50% of pregnancies.

• It results in a microcytic, hypochromic anaemia with low plasma iron, high iron-binding capacity and low serum ferritin.

• Treatment is with oral iron (e.g. 325 mg of ferrous sulfate) once to three times daily.

• Absorption is enhanced by administration of 500 mg of vitamin C.

• Side effects of iron include nausea, vomiting, abdominal pain, constipation and diarrhoea.

• Parenteral administration via intravenous or intramuscular routes is rarely required (malabsorption syndromes, severe anaemia with intolerance to oral preparations). Anaphylaxis occurs in 1%.

Folic acid deficiency

• This is much more common than vitamin B₁₂ deficiency as a cause of megaloblastic anaemia. It occurs due to poor nutrition or decreased absorption.

• Findings include macrocytic (or normocytic), normochromic anaemia with hypersegmentation of leucocytes. Reticulocyte count is normal or low. White blood cell and platelet counts are often reduced.

• It is defined as red blood cell folate level <165 ng/mL or serum folate <2 μg/L. Vitamin B₁₂ is within the normal range.

• Treatment is with oral folic acid, 1–5 mg/day.

Vitamin B₁₂ deficiency

• Deficiency is rare except in strict vegans and women with intrinsic factor deficiency.

• It may manifest as neurological defects. It is critical that women with vitamin B₁₂ deficiency not be treated with folic acid alone, as neuropathy may worsen.

• Treatment is with 1 mg of intramuscular vitamin B₁₂ daily for 1 week, then weekly for 4 weeks.

Thalassaemia

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