The Preschool Years

Structural Development of the Brain

The preschool brain experiences dramatic changes in its anatomic and physiologic characteristics, with increases in cortical area, decreases in cortical thickness, and changing cortical volume. These changes are not uniform across the brain and vary by region. Gray and white matter tissue properties change dramatically, including diffusion properties in the major cerebral fiber tracts. Dramatic increases occur in brain metabolic demands. In general, more brain regions are required in younger than in older children to complete the same cognitive task. This duplication has been interpreted as a form of “scaffolding,” which is discarded with increasing age. The preschool brain is characterized by growth and expansion that will be followed in later years by “pruning.”

Physical Development

Somatic and brain growth slows by the end of the 2nd yr of life, with corresponding decreases in nutritional requirements and appetite, and the emergence of “picky” eating habits (see Table 27.1 ). Increases of approximately 2 kg (4-5 lb) in weight and 7-8 cm (2-3 in) in height per year are expected. Birthweight quadruples by 2.5 yr of age. An average 4 yr old weighs 40 lb and is 40 in tall. The head will grow only an additional 5-6 cm between ages 3 and 18 yr. Current growth charts, with growth parameters, can be found on the U.S. Centers for Disease Control and Prevention website ( http://www.cdc.gov/growthcharts/ ) and in Chapter 27 . Children with early adiposity rebound (increase in body mass index) are at increased risk for adult obesity.

The preschooler has genu valgum ( knock-knees ) and mild pes planus ( flatfoot ). The torso slims as the legs lengthen. Growth of sexual organs is commensurate with somatic growth. Physical energy peaks, and the need for sleep declines to 11-13 hr/24 hr, with the child eventually dropping the nap (see Fig. 22.2 ). Visual acuity reaches 20/30 by age 3 yr and 20/20 by age 4 yr. All 20 primary teeth should have erupted by 3 yr of age (see Chapter 333 ).

Most children walk with a mature gait and run steadily before the end of their 3rd yr (see Table 23.1 ). Beyond this basic level, there is wide variation in ability as the range of motor activities expands to include throwing, catching, and kicking balls; riding on bicycles; climbing on playground structures; dancing; and other complex pattern behaviors. Stylistic features of gross motor activity, such as tempo, intensity, and cautiousness, also vary significantly. Although toddlers may walk with different styles, toe walking should not persist.

The effects of such individual differences on cognitive and emotional development depend in part on the demands of the social environment. Energetic, coordinated children may thrive emotionally with parents or teachers who encourage physical activity; lower-energy, more cerebral children may thrive with adults who value quiet play.

Handedness is usually established by the 3rd yr. Frustration may result from attempts to change children's hand preference. Variations in fine motor development reflect both individual proclivities and different opportunities for learning. Children who are restricted from drawing with crayons, for example, develop a mature pencil grasp later.

Bowel and bladder control emerge during this period, with “readiness” for toileting having large individual and cultural variation. Girls tend to potty “train” faster and earlier than boys. Bed-wetting is common up to age 5 yr (see Chapter 558 ). Many children master toileting with ease, particularly once they are able to verbalize their bodily needs. For others, toilet training can involve a protracted power struggle. Refusal to defecate in the toilet or potty is relatively common, associated with constipation, and can lead to parental frustration. Defusing the issue with a temporary cessation of training (and a return to diapers) often allows toilet mastery to proceed.