The symptoms and complications of thalassaemia will depend on its severity. This is determined by the number of globin genes involved and the type of thalassaemia mutations. Persons with thalassaemia trait could have mild anaemia but usually do not have symptoms or complications from thalassaemia. These persons may have only one β globin gene affected by thalassaemia (β thalassaemia trait) or at most two α globin genes affected by thalassaemia (α thalassaemia trait).Persons with non-transfusion dependent thalassaemia (NTDT) would usually have moderate anaemia and may experience intermittent symptoms of anaemia such as giddiness or breathlessness as well as intermittent transfusions. They usually experience enlargement of the liver and spleen which may sometimes cause abdominal discomfort. They may also develop other complications related to the thalassaemia, such as high blood pressure in the lungs which can cause breathlessness and decreased effort tolerance. Extra sites for production of haemoglobin outside of the bone marrow such as in the spine can cause back pain, lower limb weakness and numbness. They may also develop ankle/foot ulcers. Persons with NTDT may have both β globin genes mutations, causing only a mild decrease in the production of β globin chains. Hb H disease is when three α globin genes are mutate. Persons with transfusion dependent thalassaemia (TDT) would usually have severe life-threatening anaemia, and require regular blood transfusion for survival from less than 2 years old. Persons with TDT would usually have both β globin genes affected by severe mutations, resulting in absent or negligible β globin chains.As α globin chains are also needed to produce foetal haemoglobin, foetuses with all four α globin genes affected by thalassaemia mutations would have a very severe form of α thalassaemia, known as Bart's hydrops fetalis. Without intervention, foetuses with Bart's hydrops fetalis usually die during the pregnancy or soon after birth. The mother carrying the foetus with Bart's hydrops fetalis may also experience serious complications.