When and how does the blood develop in a fetus?

Blood development in a fetus begins around the third week of gestation, primarily in the yolk sac, where hematopoietic stem cells are first formed. By the sixth week, the liver takes over as the main site of blood cell production, followed by the spleen and eventually the bone marrow, which becomes the primary site of hematopoiesis by the end of the second trimester. This process involves the differentiation of stem cells into various blood cell types, including red blood cells, white blood cells, and platelets, essential for the fetus's growth and development.