**Core Concept**
During pregnancy, various physiological changes occur to accommodate the growing fetus. One such change involves the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and fluid balance.

**Why the Correct Answer is Right**
The correct answer is related to the increased levels of relaxin, a hormone produced by the placenta. Relaxin has a vasodilatory effect and also inhibits the renin-angiotensin-aldosterone system (RAAS), leading to decreased peripheral resistance and a subsequent decrease in blood pressure. This is a normal physiological adaptation during pregnancy to ensure adequate blood flow to the placenta and fetus. The decreased peripheral resistance also contributes to the increased cardiac output and decreased systemic vascular resistance observed during pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a correct answer. While there are various changes in the cardiovascular system during pregnancy, the correct answer is more specific to the hormonal changes.
* **Option B:** This option is not correct as it is related to a pathological condition rather than a physiological adaptation seen during pregnancy.
* **Option C:** This option is not the correct answer. Although there is a decrease in systemic vascular resistance during pregnancy, the correct answer highlights the specific hormonal adaptation responsible for this change.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the physiological changes during pregnancy can be misinterpreted as pathological conditions. For instance, a blood pressure reading of 140/90 mmHg in a pregnant woman might be considered hypertension in a non-pregnant individual, but it's actually a normal physiological adaptation during pregnancy.

**Correct Answer:** D.