The main difference between a fetus and an embryo lies in their developmental stages. An embryo refers to the early stage of development after fertilization and implantation, lasting from the moment of conception until about eight weeks in humans. During this period, the basic structures and organs begin to form.
On the other hand, a fetus is a more developed stage following the embryonic period. It starts around the ninth week after conception and continues until birth. During the fetal stage, the structures formed during the embryonic period continue to grow and mature, developing further into recognizable human form with more defined features and functionalities.