It generally consists of two types.
Primary polycythaemia: In primary polycythaemia the increase in red blood cells is due to inherent problems in their production.
Secondary polycythaemia: Secondary polycythaemia generally occurs as a response to other factors or underlying conditions that promote red blood cell production.
Red cell production (erythropoiesis) takes place in the bone marrow through a complex sequence of tightly regulated steps. The main regulator of the red cell production is the hormone erythropoietin (EPO). This hormone is largely secreted by the kidneys, although, about 10% may be produced and secreted by the liver.
Erythropoietin secretion is up-regulated in response to low oxygen levels (hypoxia) in the blood. More oxygen can be carried to tissues when erythropoietin stimulates red blood cell production in the bone marrow to compensate for the hypoxia.
Neonatal ( newborn) polycythaemia can be seen in 1% to 5% of newborns. The most common causes may be related to transfusion of blood, transfer of placental blood to the infant after delivery, or chronic inadequate oxygenation of the foetus (intrauterine hypoxia) due to placental insufficiency.