**Core Concept:** Infant growth and weight gain are crucial aspects to monitor in pediatric patients. Weight gain is a reflection of overall health and nutrition.

**Why the Correct Answer is Right:** After birth, an infant's weight doubles within the first 6 months of life. This rapid weight gain is due to increased muscle mass, fat storage, and organ development. In the first month, the infant loses some weight due to the body's initial adjustments. By the second month, the infant has regained the birth weight and then starts gaining weight. By 6 months, the infant's weight doubles from the birth weight, which indicates a healthy growth pattern and proper nutrition.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the weight of an infant does not remain constant after 6 months. It continues to increase and reaches a peak around the age of 2 years.

B. This option is incorrect because weight gain starts to slow down after the first few months of life. Doubling the weight occurs in the first 6 months, not at 9 months.

C. This option is incorrect because the weight of an infant starts to increase in the first few months, reaching a peak around the age of 2 years. Doubling the weight occurs in the first 6 months, not at 12 months.

D. This option is incorrect because the weight of an infant increases steadily from birth until the age of 2 years. Doubling the weight occurs in the first 6 months, not at 18 months.

**Clinical Pearl:** Monitoring an infant's weight gain is essential for assessing overall health and nutrition. A healthy weight gain indicates proper nutrition and development. Recognizing and addressing any concerns regarding weight gain can prevent health complications and ensure optimal growth.