Here's why:

* Yolk sac: In the early stages of fetal development, the yolk sac is the primary site of blood cell production.

* Liver: As the fetus grows, the liver takes over as the primary site of hematopoiesis (blood cell production).

* Bone marrow: By the time the fetus is near term, the bone marrow becomes the primary site of hematopoiesis and continues to be so throughout life.

The spleen does play a role in the immune system and filtering blood, but it's not involved in the production of blood cells during fetal development.