**Core Concept**
The cardiovascular system undergoes significant changes during pregnancy to accommodate the increased metabolic demands and blood volume expansion. One of the key physiological adaptations is the increase in cardiac output to meet the increased oxygen and nutrient requirements of the fetus.

**Why the Correct Answer is Right**
Pregnancy is characterized by a significant increase in maternal blood volume, which peaks at around 32-34 weeks of gestation. This increase in blood volume is accompanied by an increase in cardiac output, which is mediated by the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system. The increase in cardiac output is necessary to meet the increased oxygen and nutrient requirements of the fetus, as well as to ensure adequate perfusion of the placenta.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a decrease in cardiac output is not a physiological change in pregnancy. In fact, cardiac output increases significantly during pregnancy to meet the increased metabolic demands.

**Option B:** This option is incorrect because a decrease in blood pressure is not a physiological change in pregnancy. In fact, blood pressure typically increases during pregnancy due to the increased blood volume and cardiac output.

**Option C:** This option is incorrect because a decrease in stroke volume is not a physiological change in pregnancy. In fact, stroke volume typically increases during pregnancy due to the increased blood volume and cardiac output.

**Option D:** This option is incorrect because a decrease in heart rate is not a physiological change in pregnancy. In fact, heart rate typically increases during pregnancy due to the increased metabolic demands and the effects of the RAAS and sympathetic nervous system.

**Clinical Pearl / High-Yield Fact**
The increase in cardiac output during pregnancy is mediated by the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system, which increases blood volume and cardiac output to meet the increased oxygen and nutrient requirements of the fetus.

**Correct Answer:** A. Increase in cardiac output