**Core Concept**
During early pregnancy, a series of physical examinations can be performed to confirm the presence of a viable intrauterine pregnancy. These signs are essential in clinical practice, allowing healthcare providers to diagnose pregnancy and monitor its progression. The classical triad of 'enlarged upper part of the uterus, soft lower part of the body, and firm cervix' is a characteristic finding in early pregnancy.

**Why the Correct Answer is Right**
Hegar's sign is a clinical sign of early pregnancy, characterized by the presence of a soft, ballotable lower uterine segment (soft lower part of the body) and a firm cervix. This sign is attributed to the relaxation of the uterine musculature and the increased blood flow to the uterus, which causes the lower uterine segment to become softer. The firm cervix is a result of the increased pressure from the expanding uterine contents. Hegar's sign is typically detected around 6-10 weeks of gestation.

**Why Each Wrong Option is Incorrect**
**Option B:** Jacquemier's sign refers to a bluish discoloration of the vaginal mucosa, which is a result of increased venous pressure in the vagina due to the expanding uterus. This sign is not associated with the classical triad mentioned in the question.
**Option C:** Osiander's sign is characterized by a soft, ballotable uterus, but it does not specifically describe the classical triad of 'enlarged upper part of the uterus, soft lower part of the body, and firm cervix'.
**Option D:** Goodell's sign is the softening of the cervix, which is an early sign of pregnancy. However, it does not include the classical triad of signs mentioned in the question.

**Clinical Pearl / High-Yield Fact**
Hegar's sign is an important clinical finding in early pregnancy, but it can be difficult to detect in women with a high body mass index (BMI) or those with a retroverted uterus. It is essential to perform a thorough physical examination and consider other diagnostic modalities, such as ultrasound, to confirm the presence of a viable intrauterine pregnancy.

**✓ Correct Answer: A. Hegar's sign**