Early Embryonic Development: Germ Layers and Organogenesis

The embryo changes from a relatively simple disc into a body plan with recognizable organ systems over a short period. Developmental age is counted from fertilization, whereas obstetric gestational age is counted from the first day of the last menstrual period and is approximately two weeks ahead. Thus, developmental weeks 3 through 8 correspond roughly to gestational weeks 5 through 10. Gastrulation occurs during developmental weeks 3 and 4. Cells reorganize into three primary germ layers, establishing the basic tissues from which the embryo will be constructed. Organogenesis begins during this same developmental window and continues through week 8. By the end of the embryonic period, the major organ systems have been laid down, but they are not fully mature. Growth, refinement, and functional maturation continue during the fetal period. This timing explains why a patient may be exposed during a clinically quiet pregnancy while embryonic development is proceeding rapidly. The absence of symptoms in the pregnant person does not indicate that development is unaffected, and a later normal examination cannot establish exactly what happened at the cellular level.