## Core Concept
The respiratory system undergoes several changes during pregnancy, primarily due to the effects of progesterone and the mechanical pressure exerted by the expanding uterus. These changes can affect lung volumes, respiratory rate, and gas exchange.

## Why the Correct Answer is Right
During pregnancy, there is an increase in **tidal volume** and **respiratory rate**, leading to an increase in **minute ventilation**. This increase is largely driven by the rising levels of **progesterone**, which acts on the respiratory centers in the brain to increase the sensitivity to carbon dioxide. As a result, pregnant women tend to hyperventilate, which can lead to a decrease in **PaCO2** levels.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while there are changes in lung volumes, stating specifics without context (like which volumes change and how) can be misleading. For instance, **Functional Residual Capacity (FRC)** decreases due to the upward displacement of the diaphragm by the enlarging uterus, but **tidal volume** and **minute ventilation** increase.
- **Option B:** Without specific details on what changes are mentioned, it's hard to directly refute. However, changes in the respiratory system are well-documented and include alterations in lung volumes and gas exchange, not just a simple increase or decrease in a single parameter.
- **Option D:** Similarly, this option lacks specificity. The respiratory system does experience changes, but dismissing them as minimal or nonexistent is inaccurate.

## Clinical Pearl / High-Yield Fact
A key point to remember is that during pregnancy, **minute ventilation increases by about 40-50%** by the end of the third trimester. This increase can lead to **respiratory alkalosis** due to hyperventilation, characterized by decreased **PaCO2** levels. This change is crucial for understanding the acid-base balance in pregnant women.

## Correct Answer: C.