Erythropoiesis is the process by which red blood cells (erythrocytes) are produced. It is stimulated by decreased O2 delivery to the kidneys, which then secrete the hormone erythropoietin. This activates increased erythropoiesis in the hemopoietic tissues. In postnatal birds and mammals (including humans), this usually occurs within the red bone marrow. In the early fetus, erythropoiesis takes place in the mesodermal cells of the yolk sac. By the third or fourth month, erythropoiesis moves to the spleen and liver. After seven months, erythropoiesis occurs in the bone marrow. However, in humans with certain diseases and in some animals, erythropoiesis also occurs outside the bone marrow, within the spleen or liver. This is termed extramedullary erythropoiesis.