Growth in height and weight is slower and steadier during middle childhood (ages 7 to 12) than the preschool years. Differences between boys and girls are minimal until ages 10 to 12, when girls grow taller than boys, and boys put on extra weight but not height (Figure 44-1 and Figure 44-2). When assessing growth of children with Down syndrome and children adopted from overseas, special growth charts should be used with these populations. Girls also begin to experience pubertal changes (see Chapter 55); the average age of menarche for U.S. girls is 12 years. For boys, puberty arrives at about age 14.

During middle childhood, organ systems become more efficient and adult-like. As the gastrointestinal system matures and grows in capacity, children experience fewer stomachaches, can go longer between meals, and require fewer calories. Heart and respiratory rates decline as blood pressure increases between ages 6 and 12. Vital signs between ages 6 and 12 are as follows: pulse ranges between 65 and 100 beats per minute (average 90), there are 20 to 30 respirations per minute, and blood pressure is in the range 100-110/60-70.

The organization of the central nervous system (CNS) is like an adult’s, there are significant changes in the organization of the brain (brain development). A neurological shift in the brain occurs between the ages of 5 and 7, as increased myelination of the connections between the right and left hemispheres results in increased lateralization and specialization of areas of the brain. Also, better connections between the frontal and posterior lobes and increases in working memory prepare a child for the educational and behavioral challenges of formal schooling.

Body proportions also change. Children ages 6 to 12 have a lower center of gravity. There is an increase in leg length, a decrease in head size, and a decrease in waist size relative to height. These slender proportions contribute to physical agility, so that riding a bike or hiking a mountain trail is better coordinated. There is an increase in muscle mass relative to body weight, although muscles are still immature and remain vulnerable to injuries from overuse. Bones continue to ossify. If they are broken, bones will heal more quickly than during adolescence because they are still growing.