**Core Concept**
Pregnancy induces various physiological changes in the respiratory system to meet the increased oxygen demand and prepare for parturition.

**Why the Correct Answer is Right**
During pregnancy, the diaphragm is elevated due to the enlarging uterus, which leads to a decrease in the functional residual capacity (FRC). This decrease results from the displacement of the diaphragm upwards, reducing the volume of the thoracic cavity and the lung's ability to expand. Furthermore, the increased progesterone levels cause relaxation of the smooth muscles in the airways, leading to an increase in the airway resistance. These changes lead to a slight decrease in the forced expiratory volume (FEV1) and forced vital capacity (FVC). However, the vital capacity (VC) remains relatively unchanged.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the changes in the respiratory system during pregnancy. The decrease in FRC is a key feature, but the other parameters do not show significant changes.
**Option B:** This option is incorrect as it is not a recognized change in the respiratory system during pregnancy. While the diaphragm does move upwards, the primary effect is on the FRC and not the rib cage's circumference.
**Option C:** This option is incorrect as the changes in the respiratory system during pregnancy are primarily related to the diaphragm's elevation and the decrease in FRC, rather than an increase in lung capacity.

**Clinical Pearl / High-Yield Fact**
Pregnant women are more susceptible to respiratory depression due to the increased sensitivity to opioid analgesics and the decreased functional residual capacity.

**Correct Answer: Not Provided**