Embryo The developing human organism from fertilization through the end of week 8 of gestation. This is the embryonic period (weeks 3-8 after fertilization), during which the primordia of all major organ systems form (organogenesis) - The Developing Human: Clinically Oriented Embryology; Color Atlas of Human Anatomy Vol. 2. This is also the stage most vulnerable to teratogenic injury, since insults can cause major structural malformations or death of the conceptus - Robbins & Kumar Basic Pathology.

Fetus The developing human organism from the start of week 9 until birth. This is the fetal period, characterized not by new organ formation but by growth, maturation, and functional differentiation of the organ systems already established during the embryonic period - Color Atlas of Human Anatomy Vol. 2, "Fetal Period (Overview)." Fetal size is typically measured by crown-rump length (CRL) in cm/mm.

Key distinction The transition from embryo to fetus is gradual, but the name change matters clinically: it signals that the primordia of all major organ systems have already formed, so subsequent development is mainly growth and maturation rather than new structure formation - The Developing Human: Clinically Oriented Embryology, p. 96.

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