Fetus is the term used to describe a human in the early stages of development. More specifically, the term is used to describe the stages between the embryonic state and birth. The embryo is typically referred to as a fetus after the second month of pregnancy.
The embryo is referred to as a fetus once the initial stages of development are complete. This is typically when the embryo developed enough that it is recognizable as a small human. Once the major anatomical structures of the embryo have formed, it is referred to as a fetus. This is the nineth week of development until birth.
