**Core Concept**
Erythropoiesis is the process of red blood cell production, which is essential for delivering oxygen to tissues. During fetal development, the erythropoietic site shifts from the yolk sac to the liver and finally to the bone marrow.

**Why the Correct Answer is Right**
In gestation, the primary site of erythropoiesis changes as the fetus develops. Initially, the yolk sac produces red blood cells from approximately 2-3 weeks of gestation. However, as the fetus grows, the liver becomes the next major erythropoietic site, producing red blood cells from around 5-6 weeks of gestation until approximately 24 weeks. After this period, the bone marrow takes over as the primary site of erythropoiesis, producing red blood cells for the remainder of fetal development and into adulthood.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the yolk sac is primarily involved in early embryonic development, not gestation.
* **Option B:** Incorrect because the spleen is primarily involved in filtering the blood and storing red blood cells, not producing them.
* **Option C:** Incorrect because the lungs are primarily involved in gas exchange, not erythropoiesis.

**Clinical Pearl / High-Yield Fact**
The shift in erythropoietic sites during fetal development is essential for the fetus's increasing oxygen requirements as it grows. Understanding this process is crucial for diagnosing and managing disorders of erythropoiesis in both fetuses and newborns.

**Correct Answer:** D