**Core Concept**
Low birth weight (LBW) is defined as a newborn weighing less than 2500 grams at birth. This classification is critical in neonatal medicine as it correlates with increased risk of complications such as hypoxia, respiratory distress, and developmental delays.

**Why the Correct Answer is Right**
According to WHO and standard obstetric guidelines, a baby is classified as having low birth weight if it weighs less than 2500 grams at birth. This threshold is used globally to identify infants at risk for adverse outcomes. The 2500-gram cutoff is based on clinical and epidemiological data showing that infants below this weight have higher rates of morbidity and mortality.

**Why Each Wrong Option is Incorrect**
Option A: 1000 gm – This is far below the standard threshold and would classify the baby as very low birth weight (VLBW), not just low birth weight. It is not the accepted definition.
Option B: 1500 gm – This is also below the standard cutoff and falls into the VLBW category, not the general LBW definition.
Option C: 2000 gm – While low, this still falls below the official 2500-gram threshold and is not the standard definition used in medical literature.

**Clinical Pearl / High-Yield Fact**
Remember: **LBW = <2500 g**, **VLBW = <1500 g**. A baby weighing 2500 g or more is considered normal birth weight. This distinction is vital for neonatal assessment and early intervention planning.

✓ Correct Answer: D. 2500 gm