**Core Concept**
Hegar's sign is a clinical finding used to diagnose early pregnancy. It is characterized by a softening of the uterine isthmus, which is the area between the cervix and the body of the uterus. This softening is due to the relaxation of the uterine musculature and the formation of the decidua, a layer of cells that lines the uterus during pregnancy.

**Why the Correct Answer is Right**
The correct answer is 8 weeks because Hegar's sign can be elicited at this stage of gestation. At around 6-8 weeks of pregnancy, the uterine isthmus begins to soften due to the decidual reaction, which is an early response of the uterus to implantation of the embryo. This softening is often palpable during a bimanual examination, and it is a key clinical finding in early pregnancy diagnosis. The softening is thought to be due to the relaxation of the uterine musculature and the formation of the decidua parietalis, a type of decidua that forms in the uterine isthmus.

**Why Each Wrong Option is Incorrect**
**Option B:** 10 weeks - While the uterine isthmus may still be palpable at this stage, Hegar's sign is typically most pronounced and easily elicited at 8 weeks.
**Option C:** 12 weeks - By this stage, the uterine isthmus may be less palpable due to the growth of the uterus and the increasing size of the fetus.
**Option D:** 15 weeks - At this stage, the uterine isthmus is likely to be less palpable and Hegar's sign may not be as easily elicited.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Hegar's sign is not always present in every pregnancy, and its absence does not necessarily rule out pregnancy. However, when present, it is a useful clinical finding in the early diagnosis of pregnancy.

**✓ Correct Answer: A. 8 weeks**