fetus, term used to describe the unborn offspring in the uterus of vertebrate animals after the embryonic stage (see embryo). In humans, the fetal stage begins seven to eight weeks after fertilization of the egg, when the embryo assumes the basic shape of the newborn and all the organs are present. This stage continues until birth. The fetus is protected by a sac of amniotic fluid that also enables movement to occur. The placenta and umbilical cord are the sources of oxygen and nutrients and the means of waste elimination.

During the fetal stage, the body grows larger, the proportions of the features are refined, and organ development is completed. During the seventh and eighth weeks, the body grows more erect, the chest area develops, and the face begins to acquire a human look. In the third month, facial features continue to develop, nails form, ossification centers develop in bones, the sex of the unborn can be determined, and the fetus is capable of responding to outside stimulation. During the second trimester (fourth to sixth months), distinctive facial features develop, the fetal heartbeat can be detected, and fetal quickening (movements) can be felt externally. In the third trimester (the seventh to ninth months), the body proportions, except for the somewhat large head, are established, the skin becomes smoother, and the organs develop sufficiently for the newborn to function on its own.

If the fetus is expelled before 36 weeks of gestation are completed, it often can survive outside the womb, but artificial assistance, such as intravenous feedings and strict maintainance of the ambient temperature, may be needed during the remainder of its normal developmental period. Such births are called premature. Fetuses expelled before that period are not viable and are termed either a miscarriage or an abortion. A dead fetus delivered in the third trimester is termed stillborn.

See also reproductive system.

fetus The term used for the developing individual in the womb, by convention when it ceases to be called an embryo after about the first two months. The adjective ‘fetal’ is however often applied from an earlier stage to the cellular elements of the embryo during and after implantation in the womb, when distinguishing them from the ‘maternal’ tissue. Again by convention, the baby remains a fetus until birth, when it becomes an infant (or, for a month, a neonate). ‘Foetus’ was the traditional British spelling until recent years.

Stuart Judge

fetus (foetus) (fee-tŭs) n. a mammalian embryo during the later stages of development within the uterus; in humans it is an unborn child from its eighth week of development. f. papyraceous a twin fetus that has died in the uterus and become flattened and mummified.
—fetal adj.

fetus (foetus) Stage of embryo development in a mammal after the main adult features are recognizable. In humans, it dates from about eight weeks after conception. The abortion of a fetus continues to be an emotional debate that involves the rights of the fetus and the mother.

fetus (foetus) The embryo of a mammal, especially a human, when development has reached a stage at which the main features of the adult form are recognizable. In humans the embryo from eight weeks to birth is called a fetus.

fe·tus / ˈfētəs/ • n. (pl. -tus·es ) an unborn offspring of a mammal, in particular an unborn human baby more than eight weeks after conception.

fetus var. of FOETUS.

fetus See FOETUS.

fetus •cactus • saltus • Diophantus • Sanctus •Rastus, Theophrastus •altostratus, cirrostratus, nimbostratus, stratus •conspectus, prospectus •momentous, portentous •asbestos, Festus •apparatus, Donatus, hiatus, status •acetous, boletus, Cetus, Epictetus, fetus, Miletus, quietus •Hephaestus •Benedictus, ictus, rictus •Quintus • linctus • eucalyptus • cistus •coitus •circuitous, fortuitous, gratuitous •Hippolytus • calamitous • tinnitus •Iapetus • crepitus •precipitous, serendipitous •impetus • emeritus • spiritous •Democritus, Theocritus •Tacitus • necessitous •duplicitous, felicitous, solicitous •covetous •iniquitous, ubiquitous •detritus, Heraclitus, Polyclitus, Titus, Vitus •Pocahontas, Pontus •Plautus, tortoise •cobaltous •Duns Scotus, lotus •hostess •arbutus, Brutus •Eustace • conductus • cultus •coitus interruptus • Augustus •riotous • Herodotus • Oireachtas