**Core Concept**
During pregnancy, cardiac output increases by approximately 30-50% above pre-pregnancy values to meet the metabolic demands of the mother and the growing fetus. This increase is primarily due to a decrease in systemic vascular resistance and an increase in maternal blood volume.

**Why the Correct Answer is Right**
The increase in cardiac output is mediated by several physiological changes, including a decrease in peripheral resistance, an increase in plasma volume, and an increase in maternal heart rate. The decrease in systemic vascular resistance is largely due to the relaxation of uterine and other smooth muscles, which reduces resistance in the uterine and renal vessels. Additionally, the increase in plasma volume, which is primarily due to an increase in maternal blood volume, also contributes to the increase in cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the physiological changes that occur during pregnancy.
**Option B:** This option is incorrect because it does not specify the correct mechanism by which cardiac output increases during pregnancy.
**Option C:** This option is incorrect because it does not accurately describe the physiological changes that occur during pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the increase in cardiac output during pregnancy is a normal physiological adaptation that helps meet the metabolic demands of the mother and the growing fetus. However, in some cases, an abnormal increase in cardiac output can be a sign of preeclampsia or other pregnancy-related complications.

**Correct Answer:** D.