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term used to describe the unborn offspring in the uterus of vertebrate animals after the embryonic stage (see embryoembryo
, name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilization until it has become structurally complete and able to survive as a separate organism.
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). 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 systemreproductive system,
in animals, the anatomical organs concerned with production of offspring. In humans and other mammals the female reproductive system produces the female reproductive cells (the eggs, or ova) and contains an organ in which development of the fetus takes
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an unborn mammal during the period after its principal organs and systems have developed.

In humans, the fetal period extends from the ninth embryonic week to birth. By the ninth week of development, the fetus has taken on the external features of the human organism. The head, eyes, nose, mouth, and rudiments of the limbs can be clearly distinguished. The length of the fetus is 3-4 cm. During intrauterine life, such functions as respiration, nutrition, excretion, and metabolism are achieved by means of the placenta. By the end of the fourth month, the fetus develops a face, and movement of the limbs increases.

At five months the fetus is 25 cm long and weighs 250–300 g. The skin is red and covered with downy hair and a cheesy covering known as the vernix. Formation of the meconium occurs in the intestine. The movements of the fetus are felt by the mother; and, upon auscultation of the woman’s abdomen, the fetal heartbeat can be heard. By the end of the sixth month the fetal movements become more active. At this time, the infant can be born alive and with respiratory activity, but it usually dies shortly after birth as a result of severe prematurity and immaturity. Even at the end of the seventh month the subcutaneous layer of fat is insufficiently developed, the skin is wrinkled and densely covered with the vernix, and the body is covered with downy hair. The ear and nose cartilage is soft, and the nails do not reach the tips of the fingers and toes. In boys the testicles have not yet descended to the scrotum, and in girls the labia minora are not covered by the labia majora. The fetus may be born alive, breathing independently, but it is rarely viable.

An infant born at the beginning of the eighth month of fetal development is considered premature but is viable. However, such premature infants require special care for survival. At the end of the eighth month the fetus is 38–40 cm long and weighs 1,500–1,600 g. It is born viable but requires special care.

By the end of nine months the fetus reaches a length of 43 cm and a weight of 2,300–2,500 g. There is an increase in the subcutaneous layer of fat, the skin becomes smooth and pink, the body hair decreases, and the hair on the head becomes longer. The head is relatively large, with pronounced sutures and fontanels between the bones of the cerebral cranium. The visceral cranium is small in relation to the cerebral cranium, comprising only one-eighth of the skull (in adults, one-third). By the end of the tenth month the characteristics of prematurity completely disappear, and the infant is born.


Mnogotomnoe rukovodstvo po akusherstvu i ginekologii, vol. 1. Moscow, 1961.



The unborn offspring of viviparous mammals in the later stages of development.
In human beings, the developing body in utero from the beginning of the ninth week after fertilization through the fortieth week of intrauterine gestation, or until birth.


, foetus
the embryo of a mammal in the later stages of development, when it shows all the main recognizable features of the mature animal, esp a human embryo from the end of the second month of pregnancy until birth
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