A fetus is a stage in the prenatal development of viviparous organisms. In human development, a fetus or foetus is a prenatal human between the embryonic state and birth. The fetal stage of human development tends to be taken as beginning at the gestational age of eleven weeks, i.e. nine weeks after fertilization.

In biological terms, however, prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.