## **Core Concept**
Low birth weight (LBW) is a significant indicator of neonatal health and is defined as a birth weight of less than 2,500 grams. The etiology of LBW can be multifactorial, including maternal, fetal, and environmental factors. Understanding the primary causes of LBW is crucial for implementing preventive measures.

## **Why the Correct Answer is Right**
The most common cause of low birth weight is **premature birth** (preterm labor), which accounts for approximately 70% of all low birth weight infants. Preterm birth is defined as delivery before 37 weeks of gestation. Premature infants often have not had sufficient time to gain weight in the womb, leading to lower birth weights. This condition can result from various factors, including spontaneous preterm labor, preeclampsia, and maternal medical conditions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Intrauterine growth restriction (IUGR) is another cause of low birth weight, but it is less common than preterm birth. IUGR refers to a condition where a fetus does not grow at a normal rate inside the womb.
- **Option B:** Maternal malnutrition can contribute to low birth weight, but it is not the most common cause. Malnutrition affects fetal growth and development, potentially leading to LBW.
- **Option C:** Chromosomal abnormalities can also lead to low birth weight, but they are relatively rare compared to preterm birth.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that prevention of preterm birth is a critical strategy for reducing the incidence of low birth weight. Strategies include managing maternal health conditions, promoting adequate maternal nutrition, and reducing exposure to known risk factors for preterm labor.

## **Correct Answer:** B. Preterm birth.