An infant's physical development depends on heredity, nutrition, and environment. Physical and psychologic abnormalities can also influence growth. Optimal growth requires optimal nutrition and health.

Newborns normally lose 5 to 7% of their birth weight during the first few days of life. Newborns who are breastfeeding can lose up to 7% of their birth weight. This weight is regained by the end of the first 2 weeks as newborns start to eat more. After this, newborns typically gain about 1 ounce per day during the first 2 months, and 1 pound per month after that. This weight gain generally results in a doubling of birth weight by age 5 months and a tripling by 1 year. A newborn's length increases about 30% by age 5 months and more than 50% by 1 year.

An Infant's First Year: Physical Development During the first year of life, an infant's weight and length are charted at each doctor's visit to make sure that growth is proceeding at a steady rate. Percentiles are a way of comparing infants of the same age. For an infant at the 10th percentile for weight, 10% of infants weigh less and 90% weigh more. For an infant at the 90th percentile, 90% of infants weigh less and 10% weigh more. For an infant at the 50th percentile, 50% of infants weigh less and 50% weigh more. Of more significance than the actual percentile is any significant change in percentile between doctor's visits.

Different organs grow at different rates. For example, the reproductive system has a brief growth spurt just after birth, then changes very little until just before puberty. In contrast, the brain grows almost exclusively during the early years of life. At birth, the brain is one fourth of its future adult size. By 1 year, the brain is three fourths of its adult size. The kidneys function at the adult level by the end of the first year.

Lower front teeth usually begin to appear by the age of 5 to 9 months. Upper front teeth usually begin to appear by 8 to 12 months.

Last full review/revision November 2006 by Ruth A. Lawrence, MD