anaemia (in pregnancy)

Last edited 01/2021 and last reviewed 01/2021

The mean minimum value for haemoglobin accepted by the World Health Organisation is 11.0 g/dl (at sea level). A woman with haemoglobin levels below this value that occur during pregnancy has, by definition, anaemia in pregnancy.

Anaemia in pregnancy is more common in patients who are already anaemic at conception e.g. patients with haemoglobinopathies, poor diet, with a history of menorrhagia. Women with a multiple pregnancy are more prone to the development of anaemia.

During the antenatal period Hb estimation are routinely taken at booking, 28, 32 and 36 weeks. An iron deficiency anaemia will exhibit a low serum iron and raised total iron binding capacity, with a hypochromic microcytic film and low serum ferritin.

Iron deficiency anaemia is the most common cause of anaemia in pregnancy; it is thought that iron deficiency (ID) underlies 90% of anaemia in the UK, and 24.4% of pregnant women are estimated to be anaemic at some stage during the antenatal period (1)

• in the UK, anaemia in pregnancy is defined as haemoglobin <110 g/L in the first trimester, and haemoglobin <105 g/L in the second and third trimesters (2,3)

Treatment is dependent on the cause and severity of the anaemia

• iron deficiency anaemia in pregnancy - the finding of a microcytic hypochromic blood film (if not due to another cause such as an inherited haemoglobinopathy) should prompt iron-replacement therapy (4)
  • NICE suggest that (3):
  • pregnant women should be offered screening for anaemia. Screening should take place early in pregnancy (at the booking appointment) and at 28 weeks when other blood screening tests are being performed. This allows enough time for treatment if anaemia is detected
  • haemoglobin levels outside the normal UK range for pregnancy (that is, 11 g/ 100 ml at first contact and 10.5 g/100 ml at 28 weeks) should be investigated and iron supplementation considered if indicated
  • mild iron deficiency is treated with low-dose iron supplementation e.g. ferrous sulphate 200mg daily. Consider also giving folic acid supplementation because of possible concurrent folate deficiency
  • if there is a poor response to oral iron or oral iron is not tolerated then parenteral iron may be required. Administration should take place where resuscitation facilities are available in case of an anaphylactic reaction
• folate defiency anaemia in pregnancy - this is the second most common anaemia of pregnancy. This can be an important confounding factor in using MCV as a guide to treatment since folate deficiency causes a macrocytic anaemia and thus may 'hide' iron deficiency
  • folic acid 5mg per day is the dose of folic acid used to treat a folate-defiency anaemia
• if there is severe anaemia near to the expected date of delivery then this may need to be treated with a blood transfusion

Notes:

• physiological anaemia occurs in pregnancy because blood volume increases to a greater extent than red cell mass, thus leading to a reduction in blood viscosity and resulting in a dilutional anaemia
  • the importance of a fall in Hb during pregnancy, indicating a healthy plasma volume expansion, has been appreciated for some time (5)
• a cohort study revealed lowest perinatal mortality was associated with a lowest recorded maternal hemoglobin concentration of between 9-11 g/dL (5)
  • the authors suggested that, based on the study results, routine iron supplementation of women with a lowest Hb of 9.0 g/dL or more is unlikely to improve the perinatal mortality rate, although maternal benefits may accrue from such intervention
    • these study results are based on a generally well-nourished population and a reduced haemoglobin during pregnancy to values between 9 and 11 g/dL are more likely indicate a good response to pregnancy than true iron deficiency anemia
    • in the developing world, severe iron deficiency coupled with failure of plasma volume expansion might give similar lowest Hb but with a poor outcome, so conclusions should not be extended to their populations without further studying how to best distinguish which of these two processes predominates in any particular individual
  • the association of the birth weight being the highest (6) and perinatal mortality being the lowest when the lowest Hb during pregnancy falls to between 9 and 11 g/dL is often a surprise to those used to the original WHO definition of anaemia in pregnancy, a Hb concentration <11 g/dL
    
    

Reference:

1. Barroso F, Allard S, Kahan BC, et al. Prevalence of maternal anaemia and its predictors: a multi-centre study. European Journal of Obstetrics & Gynecology and Reproductive Biology. 2011;159(1):99-105.
2. Pavord S, Daru J, Prasannan N, et al. UK guidelines on the management of iron deficiency in pregnancy. Br J Haematol. 2019.
3. NICE (March 2016). Antenatal care for uncomplicated pregnancies
4. Doctor (August 23rd 2005): 22-23.
5. Little MP et al. Hemoglobin concentration in pregnancy and perinatal mortality.: a London-based cohort study. Am J Obstet Gynecol 2005;193:220-6.
6. Steer PJ.Maternal hemoglobin concentration and birth weight, Am J Clin Nutr 2000;71 (5 Suppl):1285S-1287S