The first year of life is characterized by a period of rapid growth and development. While your infant grows most rapidly during the first six months, he continues to grow quickly until his first birthday. Keep in mind that an infant's average weight and length is only an indication of common growth patterns. A healthy baby may not always fall within these norms. Speak to your health care practitioner if you have any concerns about your infant’s growth.
A newborn at term is about 20 inches in length, although the norm ranges between 18 and 22 inches. The average newborn weight ranges from 5 pounds 8 ounces to 8 pounds 13 ounces, according to KidsHealth.
An infant loses up to 10 percent of his birthweight in the days following his birth. He regains it quickly, returning to his birthweight in approximately seven days. Breastfed infants initially lose more weight than their bottle-fed counterparts and gain it back more slowly, according to an article in the ADC Fetal & Neonatal Edition. However, infants who are exclusively breastfed grow at a faster rate after this point, until 6 months of age when the gap in growth patterns closes.
An infant grows rapidly during his first few months of life. In general, his weight increases by 4 to 7 ounces every week and he grows in length by about 1 inch every month. Growth does not follow a strictly linear pattern for infants. For example, growth spurts are common between 6 and 8 weeks of age, when an infant is substantially hungrier to accommodate the rapid growth.
Infants 3 to 6 Months Old
Boys and girls gain weight at slightly different rates. By 3 months old, an infant boy usually weighs between 13 and 15.2 pounds and measures 23.6 to 24.7 inches in length, according to the Centers for Disease Control and Prevention (CDC). An infant girl of the same age weighs between 11.8 and 14 pounds, and measures 23 to 24.1 inches in length. By 6 months old, a baby boy weighs 16.2 to 18.8 pounds and measures 26.1 to 27.2 inches in length; whereas, a baby girl weighs 14.8 to 17.5 pounds and measures 25.3 to 26.5 inches long.
At each well-baby checkup, your health care provider weighs and measures your infant to ensure there is no major change in growth pattern -- which may suggest a nutrition or health problem that should be further evaluated.
Infants 6 to 9 Months Old
Infant growth slows once the baby is about 6 months old. Weight gain continues steadily but at a rate of approximately 3 to 5 ounces each week until 18 months. Infants continue to grow in length by about 1/2 inch every month, according to the American Academy of Pediatrics. By 9 months, an infant boy weighs between 18.2 and 21.1 pounds and measures 27.7 to 28.9 inches long. Infant girls weigh between 16.7 and 19.7 pounds and measure 27 to 28.3 inches long.
During your baby's first six months of life, she was gaining twice as much body fat as muscle. During the second half of her first year, the ratio changes and she gains more muscle than fat and begins to lose a little chubbiness.
Infants 9 Months to 1 Year Old
An infant nearly triples his birth weight by his first birthday as the rapid growth of his first year begins to slow substantially. A 1-year-old boy usually measures 29.2 to 30.5 inches in length and weighs 19.8 to 22.9 pounds, according to the CDC. A 1-year-old girl measures 28.5 to 29.8 inches in length and weighs 18.2 to 21.4 pounds.
With his new-found mobility, the infant continues to slim down, replacing some pudginess with muscle from all of his cruising and crawling, explains KidsHealth.