A fetus /ˈfiːtəs/, also spelled foetus (or archaically faetus), is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. It is also defined as the unborn young of a vertebrate, after developing to its basic form.In humans, the fetal stage of prenatal development may be defined as beginning at the eleventh week in gestational age, which is the ninth week after fertilization. In biological terms, however, prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. The use of the term "fetus" generally implies that a mammalian embryo has developed to the point of being recognizable as belonging to its own species, and this is usually taken to be the 9th week after fertilization. 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 may not all be situated in their final anatomical location.