## Core Concept
The daily extra calorie requirement during pregnancy varies by trimester. Understanding these nutritional needs is crucial for supporting the health of both the mother and the fetus. The recommended daily calorie intake increases during pregnancy to support fetal growth and maternal health.

## Why the Correct Answer is Right
During the first trimester of pregnancy, the body's calorie needs are not significantly increased compared to the second and third trimesters. The World Health Organization and other health institutions recommend that during the first trimester, the extra calorie requirement is approximately **0 kcal/day**, as the fetus's growth is not as rapid as in the later stages of pregnancy. This makes option **B. 0** the correct answer, as it accurately reflects the minimal additional calorie needs during the first trimester.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a positive value, implying an increased calorie requirement during the first trimester. While calorie needs do increase during pregnancy, this significant of an increase is not recommended for the first trimester.
- **Option C:** and **Option D:** These options also suggest increased calorie requirements. Specifically, **Option C: 300** and **Option D: 500** are more in line with recommendations for the second and third trimesters, not the first.

## Clinical Pearl / High-Yield Fact
A key point to remember is that during the first trimester, the focus is on maintaining a balanced diet rather than significantly increasing calorie intake. It's only in the second and third trimesters that a more substantial increase in calorie intake is recommended, typically by **340 calories/day** in the second trimester and **452 calories/day** in the third trimester.

## Correct Answer: B. 0