Anaemia in pregnancy is seen in around 40% of pregnant women worldwide (around 20% in the UK). Several definitions exist; the British Committee for Standards in Haematology defines it as a haemoglobin level < 110 g/l in the first trimester, <105 g/l in the second and third trimesters and < 100 g/l in the postnatal period.
Pregnancy is associated with a 20-30% increase in red cell mass. Circulating plasma volume can increase by 30-50% in single pregnancies but may double in multiple pregnancies. There is therefore a physiological dilutional ‘anaemia’. This increase in red cell mass and the needs of the developing fetus increase requirement for iron and folate, which often need to be supplemented. Normal iron absorption is around 1-2 mg a day; however, requirements may be as high as 7.5 mg a day in late pregnancy. Many women start pregnancy with depleted iron stores and a low haemoglobin concentration, and the likelihood of this increases with subsequent pregnancies.
Polycythaemia in pregnancy is rare and is usually secondary to other disease processes such as cyanotic heart disease. The underlying problem is usually more significant than the haemoglobin concentration itself. Primary polycythaemia (rubra vera) is a neoplastic disease more often seen in older patients than in the childbearing population.
Problems and special considerations
Anaemia
Normal vaginal delivery of a single fetus is associated with a blood loss of around 500 ml, but this may double with twin deliveries. Caesarean section is associated with a blood loss of 500-1000 ml. Following delivery, there is a fall in plasma volume caused by diuresis; this partially compensates for the drop in haemoglobin concentration resulting from blood loss.
Mothers who are already anaemic have less reserve than normal and may thus be more susceptible to the effects of haemorrhage. Since they rely on an even greater increase in cardiac output than normal to maintain oxygen delivery, cardiac depression (e.g. caused by general anaesthesia) may have profound effects on maternal and fetal oxygenation. Maternal myocardial ischaemia is also more common. Haemorrhage in Jehovah’s Witnesses is a particular problem and an important cause of maternal death.
Pernicious anaemia is extremely rare in pregnancy, but the presence of a macrocytic anaemia may prompt investigation. Folate and B12 deficiency may cause congenital malformations in the neonate, abruption and haemorrhage. Maternal complications include neurological complications such as subacute combined degeneration of the cord. Aplastic anaemia has been reported in pregnancy and in some cases has resolved following delivery.
Polycythaemia
There have been few published cases of polycythaemia complicating pregnancy. There may be associated thrombocytopenia or thrombocythaemia. Thrombocytopenia may be dilu- tional and may not reflect function. Thrombotic events (arterial and venous) do occur, and prophylactic aspirin and heparin have been given to prevent them.
In cyanotic heart disease, a haemoglobin concentration greater than 160 g/l is associated with poor fetal outcome. This probably represents both a marker of severity of the underlying disease and impairment of uteroplacental oxygenation resulting from increased blood viscosity.
Coagulation times may be artefactually prolonged in severe polycythaemia.
Management options
Antenatally, treatment with iron and vitamin B12 supplements is routine in pregnancy to treat and prevent anaemia. In some cases of extreme iron deficiency, parenteral iron and erythropoietin have been used.
As long as the above potential problems are considered, anaesthetic management in general is routine. Simple measures such as maintenance of normothermia and normocal- caemia may help support normal coagulation mechanisms and minimise blood loss in the intra- and postpartum periods. In anaemia, the threshold for transfusion should be lower than normal. Parenteral iron and erythropoietin have also been used postpartum, especially in Jehovah’s Witnesses - management of whom must include senior staff.
In polycythaemia, regional analgesia and anaesthesia may be precluded by recent administration of heparin, although the benefits usually outweigh the risk of epidural haematoma.
Key points
• A drop in haemoglobin concentration during pregnancy is normal.
• Postpartum diuresis partially compensates for peripartum blood loss.
• Polycythaemia in pregnancy is usually secondary to underlying disease.
Further reading
Achebe MM, Gafter-Gvili A. How I treat anemia in pregnancy: iron, cobalamin, and folate. Blood 2017; 129: 940-9.
Annamraju H, Pavord S. Anaemia in pregnancy. Br J Hosp Med 2016; 77: 584-8.
Goonewardene M, Shehata M, Hamad A. Anaemia in pregnancy. Best Pract Res Clin Obstet Gynaecol 2012; 26: 3-24.
Munoz M, Acheson AG, Auerbach M, et al. International consensus statement on the peri-operative management of anaemia and iron deficiency. Anaesthesia 2017; 72: 233-47.