At what stage during pregnancy does the embryo become a fetus?

The transition from embryo to fetus occurs at the end of the eighth week of gestation. By this time, significant development has taken place; the structures essential for life are forming, and the basic body plan is established. The term "embryo" is used from fertilization until the end of the eighth week, marking the period where major organ systems and physical features are developing.

At the beginning of the ninth week, the developing organism is referred to as a "fetus." This designation indicates that the fetus is undergoing further development and growth of these organs and systems, which will prepare it for survival outside the womb. Therefore, the correct answer is the continuation of development post-embryonic stage, as the embryo officially transitions to a fetus at the start of the ninth week of pregnancy.

The other options do not accurately represent this critical developmental milestone, as they fall either before or within the embryonic period, where the formation of vital systems is still underway.