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Phase 1: The Blueprint & Organogenesis (Weeks 3–12) The first trimester is characterized by rapid structural development. The most critical takeaway from this phase is that organogenesis (the basic formation of all organs) is completely finished by week 8.

• Nervous System: Growth begins almost immediately. The neural tube forms at week 3 and successfully closes by week 4. By week 6, the central nervous system has formed, and primitive brain waves become detectable.
• Cardiovascular System: The heart starts to beat as early as week 4 and establishes a regular rhythm by week 5. By week 6, comprehensive fetal circu ...