**Core Concept**
Fetal erythropoiesis is the process of red blood cell production in the fetus, which is crucial for providing oxygen to the developing tissues. This process occurs in the bone marrow of the fetus, starting from a specific week of gestation.

**Why the Correct Answer is Right**
Fetal erythropoiesis begins around 7-8 weeks of gestation, when the bone marrow starts to produce red blood cells. This is a critical period in fetal development, as the fetus requires a constant supply of oxygen to support its growth and development. The production of red blood cells is stimulated by erythropoietin, a hormone produced by the placenta.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too early for fetal erythropoiesis to begin. At 4-5 weeks of gestation, the embryo is still in the gastrulation stage, and the bone marrow is not yet developed.
**Option B:** This option is incorrect because it is too late for fetal erythropoiesis to begin. By 12 weeks of gestation, the fetus is already producing red blood cells, but this is not the starting point of fetal erythropoiesis.
**Option C:** This option is incorrect because it is not a specific enough answer. While it is true that fetal erythropoiesis begins during the second trimester, a more precise answer is required for the exam.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that fetal erythropoiesis is stimulated by erythropoietin, which is produced by the placenta. This hormone plays a critical role in regulating red blood cell production in the fetus.

**Correct Answer:** C. 8 weeks.