**Core Concept**
The cardiovascular system undergoes significant changes during pregnancy to accommodate the increased blood volume and cardiac output. These adaptations are essential for ensuring adequate blood flow to the fetus and the mother.

**Why the Correct Answer is Right**
The correct answer is due to the physiological changes in the maternal cardiovascular system during the last trimester of pregnancy. The increased blood volume and cardiac output lead to an upward and lateral displacement of the apical impulse. This is because the heart has to accommodate the increased volume of blood, which results in the apical impulse being felt in a more lateral and superior position in the left 4th intercostal space. This change is a normal physiological adaptation and does not indicate any underlying pathology. The increase in cardiac output is due to the high levels of progesterone, which causes vasodilation and decreased peripheral resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** Mid-diastolic murmur is typically associated with increased flow across the mitral valve, which can be seen in conditions such as mitral stenosis. This is not a normal physiological change in pregnancy.

**Option B:** Occasional atrial fibrillation is a potential complication of pregnancy, particularly in the presence of pre-existing cardiac conditions or other medical conditions. It is not a physiological change.

**Option D:** Cardiomegaly (enlargement of the heart) is not a normal physiological change in pregnancy. While the heart does work harder to accommodate the increased blood volume, it does not become enlarged.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that the physiological changes in the cardiovascular system during pregnancy are reversible and do not indicate any underlying pathology. However, any significant changes or symptoms should be investigated further to rule out any potential complications.

**✓ Correct Answer: C. Shift of apical impulse laterally and upwards in left 4th intercostal space**