Legally, life begins at birth; biologically, it begins at conception. That is why birth is merely an interruption of the developmental pattern, not the beginning of the pattern. This interruption is characterized by a graduation from the internal to the external environment. For the baby who is about to emerge from the environment in which he has lived since his life began, it is a graduation which may be easy and pleasant or fraught with so many hazards that he will fail to complete it. "In all the rest of his life there will never be such a sudden and complete change in locale. No other journey will ever start from such profound seclusion. Even in his deepest sleep, he will not be so thoroughly hidden as he is at birth".
Although the time needed to bring about the change in locale from the mother's body to the world outside is relatively short-seldom more than 48 hours even in a difficult birth—the time needed to adjust to the change is relatively long. Most babies require at least 2 weeks, and those whose birth has been difficult or premature require proportionally more time. During this period of adjustment to the postnatal environment, no marked changes in development occur.
The period of infancy is subdivided into two periods: the period of the partunate, which consists of the first fifteen to thirty minutes of life immediately after parturition, or birth, and the period of the neonate, which covers the remainder of the infancy period. During the period of the partunate, the infant ceases to be a parasite; with the cutting of the umbilical cord he becomes for the first time a separate, distinct, and independent individual. During the period of the neonate, the infant makes the adjustments essential to a life free from the protection of the intrauterine environment.
There are two indications of the difficulties the newborn infant faces in his adjustment to postnatal life. First, he loses weight -normally for approximately a week. As he becomes adjusted to his new environment, he begins to regain the lost weight and by the end of the second week of life is probably back at his birth weight. There are individual differences in this pattern of weight loss, however. Heavy infants lose more and for a longer time than light infants. First born infants generally lose less than those born later. Infants born in the summer and autumn regain their birth weight slightly sooner than those born in the winter and spring. Finally, there is a relationship between the loss of weight and the time of the first feeding. Infants fed for the first time less than 6 hours after birth lose more than infants fed somewhat later. This docs not always hold true, however; second- or later-born infants can be fed sooner than first-born, and boys sooner than girls, without the corresponding increase in weight loss.
The second indication of the difficulties a newborn infant faces in his adjustment is disorganization of behavior. All infants experience a state of relative disorganization for 24 to 48 hours following birth. Their behavior suggests that they have been stunned by the ordeal they have just experienced. How long it will take for them to recover varies; some infants require much more time than others. On the average, about a weck is needed, approximately the time needed to regain lost birth weight. Infants who lose much weight and regain it slowly are likely to take longer to achieve organized behavior.