Women often become anaemic during pregnancy because the demand for iron and other vitamins is increased. The mother must increase her production of red blood cells and, in addition, the foetus and placenta need their own supply of iron, which can only be obtained from the mother. In order to have enough red blood cells for the foetus, the body starts to produce more red blood cells and plasma. It has been calculated that the blood volume increases approximately 50 per cent during the pregnancy, although the plasma amount is disproportionately greater. This causes a dilution of the blood, making the haemoglobin concentration fall. This is a normal process, with the haemoglobin concentration at its lowest between weeks 25 and 30. The pregnant woman may need additional iron supplementation, and a blood test called serum ferritin is the best way of monitoring this.

Other causes include:
a diet low in iron. Vegetarians, and dieters in particular, should make sure their diet provides them with enough iron.
lack of folic acid in the diet, or more rarely, a lack of vitamin B12.
loss of blood due to bleeding from haemorrhoids (piles) or stomach ulcers.
anaemia is more common in women who have pregnancies close together and also in women carrying twins or triplets.