## Core Concept
The minimum alveolar concentration (MAC) is a measure used to express the potency of an inhaled anesthetic. It is defined as the concentration of vapor in the lungs that is needed to prevent movement in 50% of subjects in response to a painful stimulus, such as a surgical incision. During pregnancy, changes in physiology and the effects of anesthetics on the mother and fetus must be considered.

## Why the Correct Answer is Right
The correct answer indicates that the MAC decreases during pregnancy. This decrease is attributed to the physiological changes that occur during pregnancy, including increases in progesterone levels. Progesterone has a depressant effect on the central nervous system, which contributes to a reduced requirement for anesthetic agents. Studies have shown that the MAC of inhaled anesthetics decreases by approximately 30-40% during pregnancy. This reduction helps in understanding that pregnant women require lower concentrations of inhaled anesthetics to achieve the same level of anesthesia as non-pregnant individuals.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests that MAC increases during pregnancy, which contradicts the established understanding that physiological changes during pregnancy lead to a decreased requirement for anesthetics.
- **Option B:** This option implies no change in MAC during pregnancy, which does not align with clinical observations and research findings indicating a decrease in MAC.
- **Option D:** This option suggests a more significant decrease than what is generally accepted or recognized in clinical practice.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the MAC of inhalational anesthetics decreases during pregnancy, primarily due to the effects of increased progesterone levels. This decrease is crucial for anesthesiologists to consider when providing general anesthesia to pregnant women, as it affects the dosing of anesthetic agents.

## Correct Answer: B. Decreased.