A fetus or foetus (/ˈfiːtəs/; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo.[1] Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilisation (or eleventh week gestational age) and continues until birth.[2] Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is 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.

The word fetus (plural fetuses) is related to the Latin fÄ“tus (“offspring”, “bringing forth”, “hatching of young”)   and the Greek "φυτÏŽ" to plant. The British, Irish, and Commonwealth spelling is foetus, which has been in use since at least 1594.  The spelling with -oe- arose in Late Latin, in which the distinction between the vowel sounds -oe- and -e- had been lost. This spelling is the most common in most Commonwealth nations, except in the medical literature, where fetus is used. The more classical spelling fetus is used in Canada and the United States. In addition, fetus is now the standard English spelling throughout the world in medical journals. The spelling faetus was also used historically. In humans, the fetal stage starts nine weeks after fertilization.[9] At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown-rump, and weighs about 8 grams.[9] The head makes up nearly half of the size of the fetus.