**Core Concept**
During pregnancy, there is a significant increase in maternal blood volume to meet the metabolic demands of the mother and the growing fetus. This increase is essential for maintaining adequate blood flow to the placenta and preventing hypoxia.

**Why the Correct Answer is Right**
The increase in plasma volume is primarily due to the production of estrogen, which stimulates the release of renin and aldosterone, leading to increased blood volume and cardiac output. This increase in plasma volume is more pronounced than the increase in red blood cell mass, resulting in a mild dilutional anemia. The increase in plasma volume is most marked between 28 and 32 weeks of gestation and peaks at around 36 weeks.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as there is no significant increase in plasma volume seen in pregnancy due to dehydration.
**Option B:** This option is incorrect as the increase in plasma volume is not primarily due to increased fluid intake.
**Option C:** This option is incorrect as the increase in plasma volume is not primarily due to the increased metabolic demands of the mother.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the increase in plasma volume during pregnancy is accompanied by a mild dilutional anemia, which can be misinterpreted as iron deficiency anemia if not recognized.

**Correct Answer:** A.