fetus

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.