1. The unborn human baby when the major structures have formed: In female people, the fetus, or baby in utero, is the product of conception from the end of the eighth week to the moment of birth. Before that time, it is called an embryo.
2. The unborn young of a viviparous animal: The fetus in creatures is the embryonic period in the later stages of development when the body structures are in their recognizable forms for their species.

The fetus pertains to the latter stages of the developing young of a fauna within the uterus or within an egg.