* Yolk sac: During the early stages of pregnancy (around 3-8 weeks), the yolk sac is the primary site of red blood cell production.
* Liver: From around 6 weeks onwards, the liver takes over as the main site of red blood cell production. This continues until the third trimester.
* Spleen: The spleen also contributes to red blood cell production in the fetus, particularly in the second trimester.
* Bone marrow: By the third trimester, the bone marrow becomes the primary site of red blood cell production and continues to be the primary source throughout life.
It's important to note that the bone marrow is the only place where red blood cells are produced after birth.
