The well-known pediatrician T. Berry Brazelton once said that newborn babies are beautifully programmed to fit their parents’ fantasies and to reward the work of pregnancy. His statement seems to describe the growth pattern of the infant years. During the first year, infants grow rapidly. During the second year, the rate of physical growth slows, and there are significant gains in cognition and language.

On average, boys are bigger than girls. The data in Figure 23-1 and Figure 23-2 are based on national averages of all U.S.-born children. Separate data for different racial and ethnic groups are available from the National Center for Health Statistics. Tables developed for children who were born prematurely or who have Down syndrome are often used by pediatricians and nurses.

The average U.S. newborn weighs 3.27 kg. By the second day of life, newborns lose 5% to 10% of their birth weight, but steadily regain it by 14 days. This loss is not a problem for healthy babies, who are born with a reserve of fat that helps them get through the first few days of life until their mother’s breast milk supply is established. Infants then gain 680 g each month until 5 to 6 months of life, when their birth weight doubles. Birth weight triples by the end of the first year, to approximately 10 kg.

By age 1 year, birth height has increased by 50%. During the first 6 months after birth, height increases 2.5 cm per month from the average 50 cm at birth, and then slows during the second half of the first year. Most of this growth occurs in the trunk, not the legs. A typical 1-year-old is approximately 76 cm tall. Birth height doubles by age 4 years.