fetus

/fee"teuhs/, n., pl. fetuses. Embryol.

(used chiefly of viviparous mammals) the young of an animal in the womb or egg, esp. in the later stages of development when the body structures are in the recognizable form of its kind, in humans after the end of the second month of gestation.

Also, esp. Brit., foetus. Cf. embryo (def. 2).

[1350-1400; ME < L fetus bringing forth of young, hence that which is born, offspring, young still in the womb, equiv. to fe- (v. base attested in L only in n. derivatives, as femina woman, fecundus FECUND, etc.; cf. Gk thesthai to suck, milk, OHG taan to suck, OIr denid (he) sucks) + -tus suffix of v. action]

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Unborn young of any vertebrate, particularly mammals, after it has acquired its basic form.

In humans, this stage begins about eight weeks after conception (see embryo). The fetal stage, marked by increased growth and full development of the organ systems, climaxes in birth (see pregnancy, parturition). By the end of the third month, the arms and legs of the human fetus begin to move and reflexive movements (such as sucking) begin. Four months after conception, the fetus is about 5.3 in. (135 mm) long and weighs about 6 oz (170 g). During the fifth month, downy hairs (lanugo) cover the body and the skin becomes less transparent. At seven months, a protective greasy substance (vernix caseosa) covers the reddish, wrinkled skin. Fat is deposited under the skin during the eighth month, when the fetus typically weighs about 5 lbs (2.2 kg). A full-term fetus is about 266 days old.

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▪ embryology

also spelled  foetus 

 the unborn young of any vertebrate animal, particularly of a mammal, after it has attained the basic form and structure typical of its kind.

      A brief treatment of the fetus follows. For more information on the human fetus, see pregnancy.

      Biologists arbitrarily speak of the earliest stages of development of the fertilized egg as the embryonic (embryo) period, which ends when the external form of the embryo begins to resemble clearly the newborn of the group to which it belongs. The next period, culminating in birth, is the fetal period. In human development this transition occurs in approximately the eighth week after conception. The fetal stage is characterized by increased growth and by the full development of the organ systems.

      Some fetal disorders may cause birth of the infant before term. If spontaneous expulsion occurs before the human fetus has reached a stage of development advanced enough to allow it to live outside the womb (20 to 22 weeks), it is spoken of as an abortion or, commonly, a miscarriage. Expulsion of a dead fetus thereafter is considered a stillbirth and of a living fetus a premature birth (premature birth and postmature birth). Postmature birth is one occurring more than three weeks beyond the expected date of delivery.

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