**Core Concept**
Fetal development involves a series of complex, highly regulated processes including embryogenesis and morphogenesis. Understanding the timeline and sequence of these events is crucial for recognizing normal versus abnormal development. **Limb formation** and **growth patterns** are key aspects of this process.

**Why the Correct Answer is Right**
The correct answer is based on the principle that fetal development follows a specific sequence and timeline. **Upper limb development** indeed precedes **lower limb development**, and significant growth in length occurs during the 3rd to 5th months. The formation of thumbs and other digits is precisely regulated, occurring in specific borders of the limb buds. The statement about maximum weight gain is critical; the fetus gains the most weight in the third trimester, not the first.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because upper limb development precedes lower limb development.
**Option D:** This is incorrect because the fetus gains the most weight in the third trimester, not the first.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that fetal development is highly sequential, with each stage building upon the last. Disruptions at any point can lead to congenital anomalies. Understanding these sequences is vital for diagnosing and managing prenatal conditions.

**Correct Answer:** D. Fetus gains maximum weight in 1st trimester