The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. The head makes up nearly half of the size of the fetus. At the start of the fetal stage, the fetus is typically about 30 millimetres ( 1 + 1⁄ 4 in) in length from crown-rump, and weighs about 8 grams. In humans, the fetal stage starts nine weeks after fertilization. Ī human fetus, attached to placenta, at three months gestational age. The spelling faetus was also used historically. In addition, fetus is now the standard English spelling throughout the world in medical journals. The more classical spelling fetus is used in Canada and the United States. This spelling is the most common in most Commonwealth nations, except in the medical literature, where the fetus is used. The spelling with -oe- arose in Late Latin, in which the distinction between the vowel sounds -oe- and -e- had been lost. The predominant British, Irish, and Commonwealth spelling is foetus, which has been in use since at least 1594. The word "fetus" was used by Ovid in Metamorphoses, book 1, line 104. The word fetus (plural fetuses or feti) is related to the Latin fētus ("offspring", "bringing forth", "hatching of young") and the Greek "φυτώ" to plant. 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.
Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. In human prenatal development, fetal development begins from the ninth week after fertilization (or eleventh week gestational age) and continues until birth. Following embryonic development the fetal stage of development takes place. A fetus or foetus ( / ˈ f iː t ə s/ plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo.
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