Erythropoiesis occurs at different sites depending on the development or growth stage.  

Site of erythropoiesis before birth

 Fetal erythropoiesis begins before a person is born and its location changes as the fetus develops throughout pregnancy. 

• At week 3, erythropoiesis occurs in the yolk sac, a structure that nourishes the developing embryo.
• During the 2ndand 3rdmonth, erythropoiesis takes place in the liver and spleen of the fetus. 
• During the 5thmonth, the process occurs in the bone marrow of the fetus. 

Site of erythropoiesis after birth

After birth, erythropoiesis occurs primarily in the bone marrow, a spongy tissue found inside the bones. 

• In children, erythropoiesis occurs in the bone marrow of several different types of bones
• In adults, erythropoiesis occurs only in certain bones such as the vertebrae, breastbone, pelvis, and ribs