**Core Concept**
Peak HCG levels are seen in early pregnancy, specifically around the time of implantation, due to the trophoblastic invasion of the uterine lining and the subsequent production of HCG by the syncytiotrophoblast. HCG is a glycoprotein hormone produced by the placenta during pregnancy.

**Why the Correct Answer is Right**
HCG levels start to rise after implantation and peak around 10-11 weeks of gestation. This surge in HCG is due to the increased production of the hormone by the syncytiotrophoblast, the outer layer of the trophoblasts that invades the uterine lining. The peak HCG levels are essential for maintaining the corpus luteum, which in turn supports the early pregnancy by producing progesterone.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because HCG levels start to rise after implantation, not before.
**Option B:** Incorrect because the peak HCG levels are not seen at 6 weeks of gestation, but rather around 10-11 weeks.
**Option C:** Incorrect because HCG levels do not peak at 12 weeks of gestation, but rather around 10-11 weeks.

**Clinical Pearl / High-Yield Fact**
It is essential to note that a significant rise in HCG levels is a key indicator of a viable pregnancy, while a plateau or decline in HCG levels may indicate a non-viable pregnancy or an ectopic pregnancy.

**Correct Answer: C. 10-11 weeks.**