Final answer:
The critical transition from the embryonic stage to the fetal stage begins at about 8 weeks of pregnancy with the appearance of the fetus, which marks significant developmental milestones like the establishment of brain structures and the start of ossification.
Step-by-step explanation:
The appearance of the fetus at about 8 weeks of pregnancy signals the beginning of the fetal stage. This critical transition from the embryonic stage to the fetal stage marks a significant milestone in prenatal development. During the eighth week, multiple developmental changes take place—the head becomes nearly as large as the rest of the embryo's body, major brain structures are established, and ossification begins as bone starts to replace cartilage in the embryonic skeleton.
By the end of the eighth week, the length of the embryo is about 20 mm (0.79 inches), and the embryo weighs approximately 8 g (0.25 oz). The transition to the fetal stage involves the beginning of rapid growth and differentiation, with virtually all major body organs present, although they are not yet fully developed or functional. The fetal stage continues from week 9 after fertilization until birth, with differentiation of the reproductive organs and the establishment of organ functions being some of the key developments during this time.