**Core Concept:** First symptom of pregnancy refers to the initial signs experienced by a woman after conception, marking the start of the menstrual cycle. These symptoms can be attributed to hormonal changes occurring during early pregnancy.

**Why the Correct Answer is Right:** The correct answer, **B. Uterine Cramps**, is related to the implantation process. Implantation occurs around the 6-14th day post-conception, when the fertilized egg implants itself into the uterine wall. This process leads to mild to moderate uterine cramps, which may be mistaken for menstrual cramps by the pregnant woman.

**Why Each Wrong Option is Incorrect:**

A. **Vaginal Bleeding (Option A)**: While vaginal bleeding can occur in early pregnancy, it's not always present. A positive pregnancy test confirms pregnancy, not bleeding.

C. **Fatigue (Option C)**: Fatigue is a common symptom during pregnancy, but it is nonspecific and can occur due to various reasons. It is not the first symptom of pregnancy.

D. **Breast Tenderness (Option D)**: Breast tenderness is another common symptom during pregnancy but is not specific to early pregnancy. It can also be due to hormonal changes prior to conception.

**Clinical Pearl / High-Yield Fact:** The **Clinical Pearl** here is the importance of considering pregnancy when evaluating a woman presenting with mild to moderate uterine cramps around the expected menstrual period. A positive urine pregnancy test should be performed to confirm the diagnosis. This knowledge is crucial for healthcare providers to promptly counsel and manage the patient appropriately.

**Correct Answer:** **B. Uterine Cramps**