Key Concepts
Overview
After implantation, the embryo undergoes rapid differentiation. The embryonic period spans weeks 3–8 post-fertilization (weeks 5–10 gestational age, since GA is counted from LMP). This period is the most critical for organogenesis — when major organ systems form and are most vulnerable to teratogens. The fetal period begins at week 9 (post-fertilization) and is characterized by growth and maturation rather than new organ formation. On the exam, writers often pair stable-sounding options with unstable data—notice the mismatch before you commit. If the stem names a license or role, reread that line; scope errors are classic trap answers even when the clinical topic is familiar. Run a 60-second scan: breathing work and oxygenation, perfusion and end organs, neuro baseline, likely infection sources, and devices that can fail quietly. When two answers feel partly right, pick the one that reduces imminent harm and matches orders for the role you were given. Train yourself to state the primary risk in one short phrase before you read the options so distractors do not rewrite your priority list. On the exam, writers often pair **stable-sounding...