A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilization (or eleventh week gestational age) and continues until birth. 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. Etymology
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. The word "fetus" was used by Ovid in Metamorphoses, book 1, line 104.The predominant 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. Development in humans
Weeks 9 to 16 (2 to 3.6 months)
In humans, the fetal stage starts nine weeks after fertilization.
In contrast, precocial animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. Primates are precocial at birth, with the exception of humans.The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants. Generally speaking, fetuses of larger land mammals require longer gestation periods. The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage). In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season. See also
References
External links

Prenatal Image Gallery Index at the Endowment for Human Development website, featuring numerous motion pictures of human fetal movement. In the Womb (National Geographic video). Fetal development: MedlinePlus Medical Encyclopedia