**Core Concept**
Pregnancy induces various physiological changes to support fetal growth and maternal well-being. One such change is an increase in cardiac output to meet the increased metabolic demands of the mother and the fetus.

**Why the Correct Answer is Right**
Cardiac output increases by approximately 40% during pregnancy to meet the increased metabolic demands of the mother and the fetus. This increase is primarily due to an increase in stroke volume, which is facilitated by the increase in plasma volume and the decrease in systemic vascular resistance. The renin-angiotensin-aldosterone system also plays a role in maintaining blood pressure and cardiac output during pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood urea nitrogen (BUN) levels decrease in pregnancy due to increased renal blood flow and glomerular filtration rate, not increase.
**Option B:** Plasma osmolality decreases in pregnancy due to increased glucose and lactate levels, not increase.
**Option C:** Platelet count may decrease in pregnancy due to increased splenic sequestration, not increase.

**Clinical Pearl / High-Yield Fact**
Remember that cardiac output increases by 40% during pregnancy, which is essential for meeting the increased metabolic demands of the mother and the fetus. This increase is primarily due to an increase in stroke volume, facilitated by the increase in plasma volume and the decrease in systemic vascular resistance.

**Correct Answer:** C.