Anaemia is defined as a low concentration of haemoglobin in the blood.
Causes
The key causes of anaemia in infancy are:
- Physiological anaemia of infancy
- Anaemia of prematurity
- Blood loss
- Haemolysis
Haemolysis in an infant may be caused by:
- Haemolytic disease of the newborn (due to ABO or rhesus incompatibility)
- Hereditary spherocytosis
- G6PD deficiency
Physiological Anaemia of Infancy
There is a normal dip in haemoglobin around 6-8 weeks of age in healthy term babies. Increased oxygenation at birth provides negative feedback, suppressing erythropoietin production in the kidneys, and leading to reduced red blood cell and haemoglobin production (erythropoiesis) in the bone marrow.
Anaemia of Prematurity
Premature neonates commonly become anaemic during the first weeks of life. The more premature and unwell the infant, the more likely they are to require blood transfusions for anaemia.
Premature neonates become anaemic due to:
- Reduced iron stores (less time in utero receiving iron from the mother)
- Insufficient red blood cell creation relative to rapid growth
- Shortened red blood cell lifespan
- Reduced erythropoietin production
- Frequent blood tests (removing a significant portion of the circulating volume)
Haemolytic Disease of the Newborn
Haemolytic disease of the newborn causes haemolysis (red blood cells breaking down), anaemia and jaundice (raised bilirubin). It may be caused by incompatibility between the rhesus antigens on the surface of the maternal and fetal red blood cells. It can also be caused by ABO incompatibility (although this is often milder).
Various types of rhesus antigen can be present or absent depending on the individual. The most clinically important is the rhesus D antigen.
At times during pregnancy, such as during childbirth, fetal red blood cells may enter the mother’s blood. When a rhesus D negative woman (who lacks the rhesus D antigen on her red blood cells) becomes pregnant with a rhesus D positive fetus (who has the rhesus D antigen on their red blood cells), the mother’s immune system will recognise the rhesus D antigen as foreign and produce antibodies to the rhesus D antigen. The mother has become sensitised to rhesus D antigens.
Usually, this sensitisation process does not cause problems during the first pregnancy (unless it occurs early, such as during antepartum haemorrhage). During subsequent pregnancies, the mother’s anti-D antibodies can cross the placenta into the fetus. If that fetus is rhesus D positive, these antibodies target the fetal red blood cells, leading to haemolysis, anaemia and release of bilirubin into the blood, causing jaundice.
The direct antiglobulin test (DAT) on the infant’s blood is positive in haemolytic disease of the newborn.
Last updated March 2026
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