Newborn Infant

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Newborn Infant


an infant less than a month old. The stages of development during the newborn period differ for full-term and premature infants. A full-term baby, who undergoes an intrauterine development of ten lunar months (40 weeks, or 280 days), has an average weight at birth of 3,200–3,500 g (from 2,500 to 4,500 g). The infant has an average length of 50 cm (47–54 cm) and an average head circumference of 32–34 cm. The age, physical development, and state of health of the parents, as well as the diet and work conditions of the mother during pregnancy, influence the weight and length of the newborn infant.

The newborn period is characterized by a number of morphological and functional changes as a result of the transfer from intrauterine to extrauterine life. During the first days of life (up to four or five days) a physiological loss of weight (5 to 8 percent) occurs. A weight loss of more than 10 percent is considered pathological. Beginning on the seventh to the tenth day of life the weight is regained. During the first month of life the infant’s weight increases by 600 to 700 g.

The body temperature in the first two to three weeks is unstable and greatly depends on the temperature of the environment. Twitching and slight trembling of the limbs, as well as grimacing, may occur in the first days of life. The newborn infant has characteristic congenital reflexes that disappear by the third or fourth month. The child distinctly reacts to light and to loud sounds, and the senses of smell and taste are developed. Sometimes a lowering of reflex activity and a decrease in muscle tone are observed during the first three days of life. This usually is due to the trauma of birth and disappears as a rule by the third to fifth day.

Initially, the newborn child maintains a fetal position, with the limbs bent and held close to the trunk. The subcutaneous fat layer is distributed evenly, imparting to the infant roundness and fullness. The musculature is poorly developed, and the skin is thin and is easily injured. The head constitutes one-fourth to one-fifth of the body length. The trunk is longer than the legs, and the arms and legs are approximately the same length. The spine has no flexures. The ribs are attached to the spine at right angles, and the thorax is barrel-shaped. The large fontanel formed by the frontal and parietal bones remains open.

Respiration is uneven in frequency and depth, with 40 to 60 respiratory movements per minute. The pulse is 120 to 140 beats per minute and, during crying, 160 to 200 beats per minute. The stomach is small in capacity and lies horizontally. The intestine is relatively long and has an underdeveloped nervous apparatus, a delicate mucous membrane, an abundance of blood vessels and villi, and weak muscle and elastic layers. There is some deficiency of the intestinal glands, and the intestinal wall has great permeability. There is little saliva, and the protective function of the oral mucosa is poorly developed. All the enzymes necessary for digestion are present: amylase, ptyalin, maltase, invertase, lipase, pepsin, cathepsin, rennin, erepsin, nuclease, enterokinase, and secretin. Hydrochloric acid in both free and bound states is found in the gastric juice. Microorganisms appear in the infant’s gastrointestinal tract and respiratory system just hours after birth.

In the first two or three days of life, the first fecal matter, or meconium, is discharged. The discharge, a thick, viscous, odorless, olive-green mass, consists of mucus, bile, and desquamated epithelial cells. Later the bowels evacuate brownish green fecal matter, which is rich in mucus and sometimes is watery and foamy. On the fifth or sixth day, the normal stool, characterized by a sour odor, is established. In the first two days of life, the infant urinates four or five times per day. Beginning on the third day urination occurs more often, and, by the end of the second week, the child urinates 15 to 20 times per day.

Water metabolism plays an extremely important role in the life of the newborn child. Water constitutes 75 to 80 percent of the infant’s weight but is not firmly bound in the body. Therefore, water balance is easily disrupted. The newborn child’s daily water requirement averages 160–200 g per kg of weight.


Spravochnik pediatra. Moscow, 1966.
Tur, A. F. Fiziologiia i patologiia novorozhdennykh detei, 4th ed. Leningrad, 1967.
Molodym roditeliam, 2nd ed. Edited by V. A. Vlasov. Moscow, 1973.


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