**Question:** Fetal hemopoiesis first occurs in:

A. Liver
B. Yolk sac
C. Umbilical cord
D. Amnion

**Core Concept:** Fetal hemopoiesis refers to the production of blood cells in the developing embryo and fetus. This process occurs in specific sites where blood cells are generated in response to hematopoietic cytokines and growth factors.

**Why the Correct Answer is Right:** Fetal hemopoiesis begins in the yolk sac, which is the primary site for blood cell production in the early stages of embryonic development. The yolk sac is rich in blood vessels and allows for efficient hematopoiesis. After the yolk sac, fetal hemopoiesis continues in the liver, which becomes the major site of blood cell production as the fetus grows.

**Why Each Wrong Option is Incorrect:**

A. Liver: Though the liver continues to produce blood cells later in fetal life, it is not the initial site of hemopoiesis.
B. Yolk sac: The yolk sac is the primary site for blood cell production during the initial stages of embryonic development. Thus, this option is correct.
C. Umbilical cord: The umbilical cord is a route for blood circulation and transport, not a site for blood cell production.
D. Amnion: The amnion is the outer layer of the amniotic membrane, which provides protection and cushioning for the fetus, not a site for blood cell production.

**Clinical Pearl:** The transition from yolk sac to liver as the primary site of blood cell production is significant as it marks a critical developmental milestone in fetal hematopoiesis. This understanding is crucial for understanding the progression of blood cell production during human development and is essential for assessing fetal wellbeing through ultrasound examination.

**Correct Answer:** B. Yolk sac

**Core Concept:** The yolk sac is the primary site for blood cell production during the initial stages of embryonic development. As the fetus grows, the liver takes over as the primary site for blood cell production. This transition reflects the maturation of the fetal organism and the shift in the hematopoietic system from primitive to definitive.