**Core Concept**
Low birth weight (LBW) is a crucial indicator of neonatal health and a significant risk factor for infant mortality and morbidity. The definition of LBW is essential for tracking and addressing the health of newborns worldwide. The World Health Organization (WHO) defines LBW based on birth weight measured within the first hour of life.

**Why the Correct Answer is Right**
The WHO defines low birth weight as a birth weight of less than 2,500 grams (approximately 5.5 pounds) measured within the first hour of life. This definition is based on research indicating that newborns with birth weights below 2,500 grams are at increased risk of health complications, including respiratory distress, hypoglycemia, and infections. The measurement is taken within the first hour of life to ensure accuracy and consistency, as the newborn's weight may fluctuate shortly after birth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the WHO defines low birth weight based on a specific weight threshold, not a time frame.
**Option B:** This option is incorrect because the definition of low birth weight is not based on the time of day or the method of measurement.
**Option C:** This option is incorrect because the definition of low birth weight is not related to the gestational age of the newborn.

**Clinical Pearl / High-Yield Fact**
The WHO definition of low birth weight is crucial for identifying newborns at risk and providing timely and targeted interventions. Healthcare providers should be aware of this definition to ensure accurate identification and management of newborns with LBW.

**Correct Answer: D. 2,500 grams**