## Core Concept
The Basal Metabolic Rate (BMR) is the energy expended while at rest. During pregnancy, the BMR increases to support the energy needs of the mother and the developing fetus. This increase is crucial for the physiological changes that occur during pregnancy.

## Why the Correct Answer is Right
The BMR increases by approximately 20-30% during pregnancy. This increase starts early in pregnancy and continues throughout the gestational period. The rise in BMR is necessary to meet the additional energy demands for fetal growth, maternal physiological changes, and preparation for lactation.

## Why Each Wrong Option is Incorrect
* **Option A:** A 10% change is too low to support the increased energy demands of pregnancy.
* **Option B:** A 30-40% change might be on the higher side but 20-30% is more commonly cited.
* **Option D:** A 50% change is excessively high and not supported by physiological data.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the increase in BMR during pregnancy helps in understanding the increased caloric needs of pregnant women. This is crucial for monitoring fetal growth and ensuring the health of both the mother and the fetus.

## Correct Answer: B. 20-30%.