Where are the majority of red blood cells made?

The majority of red blood cells are produced in the bone marrow, specifically in the flat bones such as the pelvis, sternum, and ribs. This process, known as erythropoiesis, occurs from precursor cells in the marrow that differentiate into mature red blood cells. In fetal development, red blood cells are initially produced in the liver and spleen before the bone marrow takes over after birth.