<s>[INST] context: In fetus, hematopoiesis occurs in three anatomic stages.
Mesoblastic stage 

This stage of hematopoiesis begins between 10th and 14th days of gestation
It occurs in the extraembryonic structures principally in the yolk sac.
This stage of hematopoiesis essentially ceases by 10-12 week of gestation.

Hepatitis stage

By 6-8 weeks of gestation, the liver replaces the yolk sac as the primary site of gestation.
Hematopoiesis occurs in the liver throughout the remainder of the gestation and the liver remains the predominant hematopoietic organ, till 20-24 weeks of the gestation.
After this, the hematopoiesis begins to decrease in liver and in full-term infant hepatic hematopoiesis is seen only in widely scattered small foci which soon become inactive after birth.

Myeloid stage

Hematopoiesis in the marrow starts by the 4th month of gestation.
By birth, marrow throughout the skeleton is hematopoietically active and virtually the sole source of blood cells.

Hematopoiesis after birth
Upto puberty - Hematopoiesis occurs throughout the skeleton
After puberty (By the age of 18) - Only the vertebrae, ribs, sternum, skull, pelvis and proximal epiphyseal regions of the humerus and femur retain red marrow, the remaining marrow becomes yellow, fatty an inactive. Thus, in adults only about half of the marrow space is active-in hematopoiesis. [/INST] question: Which organ is the primary site of hematopoiesis in the fetus before mid-pregnancy – | options: A: Bone B: Liver C: Spleen D: Lung | answer: B </s>