A fetus also spelled foetus or, archaically, faetus, is the term used to refer to a prenatal mammal or other viviparous vertebrate between its embryonic state and its birth. The 2008 Britannica Concise Encyclopedia defines it as the "[u]nborn young of any vertebrate, particularly mammals, after it has acquired its basic form."
In humans, the fetal stage of prenatal 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. 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; this is the point usually taken to be the ninth 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 some not yet situated in their final anatomical location.