**Core Concept**
Pregnancy is a state of dynamic physiological adaptation to accommodate the growing fetus, leading to changes in maternal cardiovascular, respiratory, and renal systems. One of the key adaptations is a significant increase in maternal blood volume, which is essential for meeting the increased oxygen and nutrient demands of the fetus.

**Why the Correct Answer is Right**
During pregnancy, there is a marked increase in cardiac output, which is achieved by a combination of increases in heart rate and stroke volume. This increase in cardiac output is necessary to meet the increased metabolic demands of the growing fetus. Additionally, there is an increase in intravascular volume, which helps to maintain blood pressure and perfusion of vital organs. However, peripheral vascular resistance actually decreases during pregnancy, not increases. This decrease is mediated by the vasodilatory effects of progesterone and other factors, which help to reduce blood pressure and improve placental perfusion.

**Why Each Wrong Option is Incorrect**
**Option A:** While it is true that intravascular volume increases during pregnancy, this is not the least likely change. In fact, it is a critical adaptation to ensure adequate perfusion of the placenta and fetus.

**Option B:** Increase in cardiac output is a well-established physiological change during pregnancy. It is essential for meeting the increased metabolic demands of the growing fetus.

**Option C:** Increase in stroke volume is another key adaptation during pregnancy, contributing to the overall increase in cardiac output.

**Clinical Pearl / High-Yield Fact**
It is essential to note that while peripheral vascular resistance decreases during pregnancy, there is a compensatory increase in systemic vascular resistance after delivery, which can lead to postpartum hypertension in some women.

**Correct Answer:**
✓ Correct Answer: D. Increase in peripheral vascular resistance