**Core Concept**
The Hegar sign is a clinical finding used in obstetrics to assess the status of pregnancy. It is characterized by the softening of the lower uterine segment, which can be felt on palpation, resulting from the relaxation of the uterine muscles in response to the presence of an intrauterine pregnancy.

**Why the Correct Answer is Right**
The Hegar sign is typically observed earliest in the first trimester of pregnancy, around 6-8 weeks post-conception. This occurs due to the relaxation of the uterine muscles, allowing for the softening of the lower uterine segment. The softening is a result of the hormonal changes caused by the early pregnancy, specifically the increase in progesterone levels. The relaxation of the uterine muscles facilitates the growth of the embryo and the development of the placenta.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Hegar sign is not typically observed in early pregnancy, around 4 weeks post-conception. At this stage, the uterine muscles are still relatively firm.
**Option B:** This option is incorrect because the Hegar sign is not a characteristic finding in pseudocyesis (false pregnancy). In pseudocyesis, the uterine muscles may be firm or irregular, but not soft.
**Option C:** This option is incorrect because the Hegar sign is not a specific finding in ectopic pregnancy. While ectopic pregnancy can cause uterine tenderness, the Hegar sign is not a reliable indicator of this condition.

**Clinical Pearl / High-Yield Fact**
The Hegar sign is an important clinical finding in obstetrics, allowing healthcare providers to assess the status of pregnancy and rule out other conditions such as pseudocyesis or ectopic pregnancy.

**Correct Answer:** C. 6-8 weeks post-conception.