**Core Concept:**
The question is asking about the growth of a newborn baby, specifically focusing on doubling its birth weight. In the context of human fetal development, birth weight is the weight of a newborn at the time of birth. The growth trajectory of a fetus is influenced by various factors, including genetics, nutrition, and overall maternal health.

**Why the Correct Answer is Right:**
The correct answer is "C. At birth". During the last trimester of pregnancy, the fetus undergoes rapid growth and development. This period is crucial for the baby's growth, as it is when organs, tissues, and body systems develop and mature. At birth, the baby's weight doubles from its initial value, which is usually around 3-4 kg for a healthy newborn. This is a result of the baby's physiological adaptation to life outside the uterus, including increased muscle mass, fat deposition, and lung maturation.

**Why Each Wrong Option is Incorrect:**
A. "Weeks" does not accurately represent the specific time period during pregnancy when the baby's weight doubles. While growth does occur throughout pregnancy, the doubling of weight mainly takes place during the last trimester.
B. "Days" is too short a time frame to account for the significant weight increase that occurs in the last trimester.
D. "Weeks" is too broad a term, not specifying the exact time when the weight doubles.

**Clinical Pearl:**
The doubling of birth weight is a notable milestone in human fetal development. It is essential to understand the impact of this period on the baby's growth and overall health. This concept helps medical professionals assess the overall fetal growth and development during prenatal visits, ensuring proper monitoring and intervention when necessary.

**Why This is Relevant:**
Understanding fetal growth and development, including the doubling of birth weight, is crucial for healthcare professionals, particularly obstetricians and pediatricians. They utilize this knowledge to assess fetal well-being, diagnose potential complications, and provide appropriate interventions to ensure the proper growth and development of the fetus during pregnancy. This understanding is vital for delivering healthy babies and ensuring optimal postnatal care.