Infancy describes the growth and development of the child from about the fourth week through the second year of life. The infant experiences rapid physical growth during this time. The birth weight doubles by the fifth month and triples by the end of the first year, and the infant gains about 2,300 to 2,700 gm (5 to 6 lb) per year for the next several years (Puckett & Black, 2004). The infant grows approximately 25 cm (10 in) by the end of the first year, an average of 13 cm (5 in) by the end of the second year, and 7 cm (3 in) the following year (Deiner, 1997). In addition to weight and length, head circumference is an important physical feature to measure at regular intervals. Changes in head circumference are important because they denote brain growth. During the first year of life, head circumference increases from 33 or 36 cm (13 or 14 in) at birth to about 43 or 46 cm (17 or 18 in), with most of this growth occurring during the early months of development. The circumferences of the chest and abdomen are about the same in the neonate, but during infancy the chest circumference becomes larger than that of the abdomen. Deciduous, or primary, teeth appear at about 6 to 8 months and continue to erupt until all 20 are in place by toddlerhood. The skeletal structure of the infant hardens, and the musculature increases in weight and density. Typically, African American children show more rapid skeletal growth than white children do, and the bones of females generally grow faster than those of males (Puckett & Black, 2004; Tanner, 1990).

Another manifestation of infancy is the apparent disappearance or, more accurately, the integration of many of the primitive reflexes into the baby’s developing nervous system. This occurs as the cerebral cortex matures and begins to exert control over the lower central nervous system. This control is accomplished, in part, by a process called myelinization. Myelin is a soft, white, fatty substance that coats and protects many nerve cells. It allows for rapid transmission of neural messages from the brain to other parts of the body. Myelinization begins in utero at about the fourth gestational month, with some neural pathways (e.g., the brain stem) being fully myelinated by the 30th gestational week (Amand, Phil, & Hickey, 1987); however, myelinization is not complete at birth (Biese & Wang, 1994). Although it will not be complete until adulthood, by 6 months many of the cortical fibers have been sheathed with myelin, thus facilitating greater cortical control and enabling the infant to achieve various developmental milestones such as sitting up, grasping, and walking, and various cognitive and adaptive skills necessary for maturation (Tanner, 1990).