A newborn doubles his height at bih by what age?
**Core Concept**
The rapid growth of a newborn, known as the growth spurt, is a critical period in pediatric development. This growth spurt is characterized by a significant increase in height and weight, primarily due to the maturation of the hypothalamic-pituitary-adrenal (HPA) axis and the growth hormone-insulin-like growth factor 1 (GH-IGF-1) axis.
**Why the Correct Answer is Right**
The newborn growth spurt is a result of the increase in growth hormone (GH) production, which stimulates the production of insulin-like growth factor 1 (IGF-1). IGF-1, in turn, promotes the growth and differentiation of cells, particularly in the bones, muscles, and skin. By around 4-6 months of age, the growth rate begins to accelerate, and the infant doubles their birth length. This rapid growth is essential for the development of the infant's physical and cognitive abilities.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the growth spurt typically begins around 4-6 months of age, not immediately after birth.
* **Option B:** This is incorrect because the growth rate does not double the birth length until around 4-6 months of age.
* **Option C:** This is incorrect because the growth spurt is a result of the increase in GH and IGF-1 production, not just the maturation of the HPA axis.
**Clinical Pearl / High-Yield Fact**
It's essential to note that the growth rate of an infant is a critical indicator of their overall health and development. A decrease in growth rate can be an early sign of underlying medical conditions, such as nutritional deficiencies or hormonal imbalances.
**Correct Answer: C.**