Infancy Physical Development: Average Growth
Babies grow at an amazing rate in the first months and years of life as they rapidly reproduce cells and grow in length and weight. In the first 2 years, babies grow to almost half their adult height and can quadruple their birth weight. During this period, it's important for caregivers to take their infants to the pediatrician for well-baby checkups (during which they will be weighed and measured) on a regular schedule to make sure they are growing at the appropriate rate. During the first year, babies will continue to increase their level of body fat. This "baby fat" allows a baby to maintain their body temperature. As babies grow in size and begin to build muscle, this baby fat will begin to disappear.
In the first two years of life, a growing child's bodily proportions also change. When infants are born, most of their body mass is in their head. As they grow older, the rest of their bodies catch up. Just as they develop their motor skills from the center of the body outward and from their head to their feet, they also grow and gain mass in that order. Babies grow first in their chest and trunk and then in their arms and legs. Over the first year of life, babies' bones and skeletons ossify, or harden. When babies are born, their bones are softer and more like cartilage. This allows them to be flexible, fit inside the mother's womb, and pass through the birth canal. However, as their bones harden in the first year, the skeleton is better able to support their weight during activities such as crawling and walking. Babies also have "soft spots" in their skull because some parts of the skull haven't fused together yet. By age 2 years, babies' skulls are as hard as adult skulls, but in the first months, caregivers need to be careful how they handle the baby and protect their heads.
As noted before, infants grow exponentially in the first 2 years. In the first 3 months, they grow up to 2.5 inches and 3 pounds. Between the ages 4 to 6 months, they grow another 2.5 inches and gain an average of 4 pounds. Between 7 and 9 months, they grow an average of 2.5 inches and 4 pounds. Between 10 and 12 months, they grow another 2.5 inches and another 3 pounds. During the second year, toddlers grow about 1 inch and 2 pounds about every 3 months. Children's growth slows considerably after age 2 years.