## **Core Concept**
Hematopoiesis, the process of blood cell formation, transitions through various sites during fetal development. Initially, it occurs in the yolk sac, then shifts to the liver and other sites before finally localizing in the bone marrow.

## **Why the Correct Answer is Right**
By the third month of gestation, the primary site of hematopoiesis has shifted from the yolk sac (where it initially occurs) to the liver. The liver becomes a major hematopoietic site during the second and third months of gestation. This is because the bone marrow, which will eventually become the main site for blood cell production, is not yet fully developed and functional at this stage.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The yolk sac is the earliest site of hematopoiesis but its role diminishes by the third month of gestation.
- **Option B:** While the spleen can have some hematopoietic activity, it is not the primary site during the third month.
- **Option D:** The bone marrow starts to show hematopoietic activity but is not the primary site of hematopoiesis during the third month of gestation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that hematopoiesis occurs in a sequence of sites during fetal development: first in the **yolk sac** (early weeks), then the **liver and spleen** (from about the second to fifth month), and finally in the **bone marrow** (which becomes the lifelong site).

## **Correct Answer:** . Liver.