**Core Concept**
During pregnancy, there are various cardiovascular changes that occur to adapt to the increasing demand for oxygen and nutrients. One of these changes is the slight left axis deviation in the electrocardiogram (ECG), which is caused by the increase in plasma volume and cardiac output.

**Why the Correct Answer is Right**
The slight left axis deviation in ECG is due to the increased plasma volume and cardiac output during pregnancy. This leads to a slight increase in the left ventricular mass, which in turn causes a shift in the axis of the heart. Additionally, the increased cardiac output also results in a slight increase in the left ventricular end diastolic pressure, which contributes to the left axis deviation. This change is a normal adaptation to pregnancy and is not a cause for concern.

**Why Each Wrong Option is Incorrect**
* **Option A:** Slight right axis deviation in ECG is not a common finding in pregnancy and can be a sign of underlying cardiac disease.
* **Option C:** Diastolic murmurs are not typically associated with normal pregnancy and can be a sign of cardiac abnormalities such as hypertension or cardiac disease.
* **Option D:** Pulse rate is actually increased during pregnancy due to the increased cardiac output and plasma volume.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cardiovascular changes during pregnancy are normal adaptations and are not a cause for concern unless accompanied by symptoms such as chest pain or shortness of breath.

✓ Correct Answer: B. Slight left axis deviation in ECG