Babyhood is one of the two periods of rapid growth during the life span; the other comes at puberty. During the first six months of life, growth continues at the rapid rate characteristic of the prenatal period and then begins to slow down. In the second year, the rate of growth decelerates rapidly. During the first year of life, the increase in weight is proportionally greater than the increase in height; during the second year, the reverse is true.
If the rapid growth characteristic of the prenatal and early postnatal periods did not decelerate soon after birth, the child would grow into a giant. It has been estimated that if weight increased at the same rate it did during the first year of life, a child who weighed seven pounds at birth would weigh 230,029 pounds at eleven years of age.
While the general pattern of growth and development is similar for all babies, there are variations in height, weight, sensory capacities, and other areas of physical development. Some babies start life smaller and less well developed than the norm. This may be due to prematurity or to a poor physical condition resulting from maternal malnutrition, stress or some other unfavourable condition during the prenatal period. As a result, such babies tend to fall behind their age-mates during the babyhood years.
The pattern of physical growth in babyhood is much the same for boys and girls. However, within the sex groups there are marked variations. Throughout the first year of life, there is little difference in height and weight between black and white babies of comparable economic levels. Differences begin to appear in the second year, however, because black children are, typically, of a more slender build than white children.
There are also variations in body size of babies of different socioeconomic levels. Babies whose parents are of the lower socioeconomic levels tend to be smaller, in both weight and height, than those whose parents come from the higher socioeconomic levels. Body build, which begins to be apparent during the second year of life, also contributes to variations in height and weight.
Throughout the babyhood period variations not only continue but become more pronounced. At all times variations in weight are greater than variations in height. This is because variations in weight are dependent partly on body build and partly on eating habits and diets.
However, in spite of variations in physical growth and development, it is possible to get a general picture of the pattern of growth and development during the babyhood years.
At the age of four months, the baby’s weight has normally doubled. At one year, babies weigh, on the average, three times as much as they did at birth, or approximately 21 pounds. At the age of two, the typical American baby weighs 25 pounds. Increase in weight during babyhood comes mainly from an increase in fat issue.
At four months, the baby measures between 23 and 24 inches; at one year, between 28 and 30 inches; and at two years, between 32 and 34 inches.
Head growth slows down in babyhood, while trunk and limb growth increases. Thus the baby gradually becomes less top-heavy and appears more slender and less chunky by the end of babyhood.
The number of bones increases during babyhood. Ossification begins in the early part of the first year, but is not completed until puberty. The fontanel, or soft spot on the skull, has closed in approximately 50 percent of all babies by the age of eighteen months, and in almost all babies by the age of two years.
Muscles and Fat
Muscle fibers are present at birth but in very undeveloped forms. They grow slowly during babyhood and are weak. By contrast, fat issue develops rapidly during babyhood, due partly to the high fat content of milk, the main ingredient in a baby’s diet.
During the second year of life, as body proportions change, babies begin to show tendencies toward characteristic body builds. The three most common forms of body build are ectomorphic, which tends to be long and slender, endomorphic, which tends to be round and fat, and mesomorphic which tends to be heavy, hard, and rectangular.
The average baby has four to six of the twenty temporary teeth by the age of one and sixteen by the age of two. The first teeth to cut through are those in the front, the last to appear are the molars. The last four of the temporary teeth usually erupt during the first year of early childhood.
At birth, brain weight is one-eighth of the baby’s total weight. Gain in brain weight is greatest during the first two years of life, thus accounting for the baby’s top-heavy appearance. The cerebellum, which plays an important role in body balance and postural control, triples in weight during the first year of postnatal life. This is true also for the cerebrum. Immature cells, present at birth, continue to develop after birth but relatively few new cells are formed.
Sense Organ Development
By the age of three months, the eye muscles are well-enough coordinated to enable babies to see things clearly and distinctly and the cones are well-enough developed to enable them to see colors. Hearing develops rapidly during this time. Smell and taste, which are well developed at birth, continue to improve during babyhood. Babies are highly responsive to all skin stimuli because all sense organs relating to touch, pressure, pain, and temperature are present in well-developed forms.