INFANCY

Growth and Physical Development

Immediately after birth, infants lose approximately 6 percent of their body weight because of fluid loss and some breakdown of tissue. Infants usually regain their birthweight by 10 to 14 days after birth, and thereafter weight gain proceeds at a slower rate. Typically, infants' birthweight doubles by 4 to 6 months and triples by age 1. On average, infants gain 5 to 7 oz per week in the first 4 to 6 months and 3 to 5 oz per week from 6 to 18 months. Infants usually increase in length by 50 percent in the first year, but the rate of increase slows down during the second half of the year. From birth to 6 months, infants gain approximately 1 inch per month, and from 6 to 12 months of age, they gain about a 1/2 inch per month.

These enormous changes make it difficult for infants to control their rapidly growing bodies. In addition, the head and trunk constitute about 70 percent of infants' body length and most of its mass, making it difficult for them to achieve upright postures or any form of locomotion. However, this large mass offers infants weight-training opportunities. As infants roll over, crawl to get a favorite toy, or walk along furniture, their head and trunk may be difficult to control, but they benefit from the physical challenge by increasing motor control and muscle development.

Muscles are not well developed at birth; therefore, most of the first year of life involves building muscle mass. Infants need arm muscles to pick up objects. They need leg muscles to crawl, stand, and eventually walk. They need their head and trunk muscles to control these two large body parts. And they need arm and leg muscles to move their trunks.

Neural development in infants involves the connection of 200 billion neurons, the conducting cells of the nervous system. Stimulation is critical to neural development. Without stimulation (e.g., visual, auditory, tactile), these cells die. With stimulation, these cells create elaborate networks that promote sensory and motor skill development. Stimulation can be provided in many ways (e.g., by hanging a mobile over the infant's crib, which may motivate the infant to reach, or by placing a toy just out of reach, which may motivate the infant to roll over).