**Core Concept**
Fetal development involves a complex series of cellular, tissue, and organ transformations from a single fertilized cell to a fully formed infant. This process is influenced by genetic factors, hormonal regulation, and environmental stimuli. Understanding the timing, sequence, and mechanisms of fetal development is crucial for identifying and managing potential complications.

**Why the Correct Answer is Right**
The correct answer relates to the critical period of fetal development, specifically the process of gastrulation, when the embryo undergoes a series of complex cellular rearrangements to form the three primary germ layers: ectoderm, mesoderm, and endoderm. These layers eventually give rise to all tissues and organs in the fetus. Accurate timing and completion of gastrulation are essential for proper fetal development.

**Why Each Wrong Option is Incorrect**
* **Option A:** This statement may be true, but its relevance to fetal development is unclear without further context. Fetal development involves numerous critical periods, and this option may not specifically address the question.
* **Option B:** This statement could be misleading, as the development of the heart and major vessels occurs relatively early in fetal development, but the process is indeed complex and involves multiple stages.
* **Option C:** This statement is false, as the development of the brain and nervous system is a highly complex and prolonged process that spans much of fetal development.

**Clinical Pearl / High-Yield Fact**
The critical period of gastrulation (approximately 3-4 weeks post-fertilization) is a key event in fetal development, marking the transition from a simple, bilaterally symmetrical embryo to a more complex, trilaterally symmetrical fetus. Abnormalities during this period can lead to severe developmental defects.

**Correct Answer:** D.