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.

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.

Learn more about fetus with a free trial on Britannica.com.