5.1 Physical Health and Growth in Early Childhood

Physical Proportions and Height

In the United States, on average, children gain between 2 and 3 inches (approximately 5–7.5 cm) in height and 4 to 5 pounds (approximately 1.8–2.25 kg) in weight per year between ages three and six (MedlinePlus, 2022) (Figure 5.2). A key change during this period is the development of more adult-like body proportions. The head accounts for a steadily decreasing proportion of body length throughout the childhood years (recall the cephalocaudal principle). Legs grow proportionally longer, allowing children to become more mobile, more agile, and better able to balance and coordinate their movements. While three-year-olds tend to have cylindrical body shapes, with little definition between the waist and hips, the average six-year-old has a more hourglass-like shape. This change will eventually become more marked for girls, who will develop wider hips and narrower waists at puberty. At six years of age, the average child in the United States is 45 inches tall (approximately 115 cm) and weighs 45 pounds (approximately 20.4 kg) (Centers for Disease Control and Prevention [CDC], 2023).

Figure 5.2 The relative growth comparison between a three-year-old and six-year old child, such as these siblings, shows some aspects of the physical changes that occur across these early childhood years. (credit: “Siblings: 3 and 6 years old” by Eric & Laurel Glenn/Flickr, CC BY 4.0)

Evidence indicates that the full height potential of our genetic inheritance is affected by several environmental influences. Economic disadvantage, physical neglect, disease, and malnutrition can affect biological and epigenetic mechanisms associated with height, which can take several generations to return to the normal range (Bogin, 2013; Lang et al., 2019; Simeone & Alberti, 2014). These influences may partly explain generational variations in height.

One directly observable consequence of malnutrition is stunting, or impaired growth in height (Figure 5.3). Stunting, which cannot be reversed and can increase risks for cognitive and physical damage, has become a global concern (De Sanctis et al., 2021; Bhutta et al., 2020). Particularly in war-torn, severely impoverished, and drought-stricken areas, inadequate nutrition is widespread, and stunting is an inherent risk. As a result, an estimated 149 million children under five, or more than 20 percent of the global under-five population, undergo stunting. In some poor parts of Africa and southern Asia, the proportion rises to nearly half (UNICEF/World Health Organization [WHO]/World Bank Group, 2021).

Figure 5.3 Nutrition plays a significant role in stature and overall physical growth. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

In contrast, when children receive sufficient nutrition, height differences are largely dictated by genetic and ethnicity factors. For example, genetic contributions to height are similar across geographic regions, and children descended from the geographic-cultural region of Europe tend to be taller than children descended from the geographic-cultural region of East Asia, regardless of where they live today, illustrating the strong influence of genetics and ethnicity (Jelenkovic et al., 2016).¹ Additionally, socioeconomic factors play a role in determining differences in height and weight. Children in high-income countries who receive better nutrition and health care grow taller and weigh more than their peers in low- and middle-income countries (Reynolds et al., 2017).

Sex Differences in Growth

During early childhood, boys start becoming taller and heavier on average than girls. Though still relatively minor, differences are more pronounced than in infancy and toddlerhood. For example, according to the World Health Organization (WHO), the average healthy four-year-old boy is about 0.75 inch (2 cm) taller and about 2 pounds (1 kg) heavier than the average healthy girl. At this age, girls are already developing more fatty tissue and boys have comparatively more muscle (McCarthy et al., 2014). This variation is partly due to differences in hormone production between the sexes and are probably genetically programmed.

Even in the first few months of life, male and female infants appear to store fat differently, perhaps due to different levels of hormone production (Davis et al., 2019). However, sex-related growth changes in early childhood may also be influenced by external factors like diet and exercise. Regardless of how they originate, these developmental differences become more apparent in large motor activities, which we’ll discuss in 5.2 Motor Development and Physical Skills in Early Childhood. In early childhood, however, physical differences that may exist between boys and girls do not have any major practical applications.

In any case, the variance within each sex is much larger than that between the sexes. That is, height and weight vary widely among all boys and among all girls, but the average difference between boys and girls is very small. What are considered "normal" heights and weights for both boys and girls also varies. As always, group averages mask large individual differences, and individual variation is normal.

Neurodevelopmental Growth

The brain both orchestrates and complements the growth of the body. That is, neural growth allows the development and expansion of new competencies, and as experience and practice with competencies increase, the brain responds by growing new connections. This process occurs throughout the lifespan but is particularly pronounced in early childhood. By the age of six, a child’s body weight is only about 30 percent of the average normal adult weight, but the brain has reached 90 percent of its adult size (Stiles & Jernigan, 2010) (Figure 5.4). The increased mass is accounted for by the growth of axons, dendrites, and synaptic connections that allow for more complex neural and motor processing. A more substantial proportion of the increased brain weight is attributable to increased myelination. This development allows for faster and more coordinated activity, as well as advances in learning and memory (Chevalier et al., 2015; Deoni et al., 2016; Gilmore et al., 2018).

Figure 5.4 The brain grows significantly during early childhood, both in size and in its structures and connections. (credit "infancy": modification of work "2900 Photo of Neonate-02" by OpenStax Anatomy & Physiology/Wikimedia Commons, CC BY 4.0; credit "5 Years": modification of work "Walk By the Sea" by Patrick Metzdorf/Flickr, CC BY 2.0; credit "adulthood": modification of work "the optimist" by Erich Ferdinand/Flickr, CC BY 2.0; attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

For the most part, maturation dictates the timing of myelination and brain development. Recall from 3.1 Physical Development in Infants and Toddlers that myelination is the process of wrapping axons in myelin to speed neural transmissions. The myelination of neurons that are essential to early physical development is mostly complete by 40 months of age. However, when life experiences are limited or the environment is highly stressful, myelination and overall brain growth will be slowed.

From an early age, the amount of stimulation a child receives has a significant effect on brain structure, weight, and volume (Gilmore et al, 2018; Lawson, 2013; Ziegler et al., 2020). Poor nutrition may also lead to deficits in myelin development and a general decrease in brain mass. However, the brain remains quite plastic in early childhood, and early intervention can often reverse these negative effects (de Faria et al., 2019).

The brain continues to overproduce dendrites and synapses, which then leads to additional synaptic pruning. This alone tells us that brain plasticity remains strong, and that experiences and opportunities have roles in development. A cross-sectional study with participants ranging in age from newborn to 91 years found that the density of dendrites among children in early childhood was two to three times that of the average adult (Petanjek et al., 2011). This indicates that a wealth of growth and learning are unfolding in early childhood, particularly for children in environments rich with learning and play experiences.

Growth patterns include experience-expectant processes, which are universal and occur in all individuals as part of typical development. They include the brain's being ready to receive and process sensory input, such as vision and language, during critical periods of development. In contrast, experience-dependent processes rely on the brain's ability to adapt and rewire in response to specific, individual experiences, such as learning a new language or a musical instrument. These experiences are unique to each person and are influenced by the person’s environment, learning opportunities, and social interactions. Thus, the brain appears programmed to expand.

The process of overproduction and pruning of synapses probably doesn’t stop until the third decade of life, especially in the prefrontal cortex (Kolk & Rakic, 2022). That the brain continually reorganizes itself tells us that a person’s environment has a persistent influence on outcomes. For instance, the onset of some neuropsychiatric disorders, like schizophrenia, may result from (still unidentified) environmental factors acting on genetic information.

Psychologists already know that environmental factors are associated with some areas of brain development. For instance, low socioeconomic status and poor nutrition are linked to less brain growth and less activation of specific brain regions. These findings likely have implications for school readiness, higher-order thinking, memory, and self-regulation (Hung et al., 2023; Moriguchi & Shinohara, 2019; Tomasi & Volkow, 2021). In another example, parents who respond with greater compassion and use fewer harsh tones exhibit the warmth and sensitivity associated with the development of more advanced brain networks in children (Suffren et al., 2022). It’s also likely that overarching factors—like overall intellectual stimulation, reduced levels of stress, and better nutrition—account for a proportion of the differences in brain development (Copeland et al., 2021; Kopala-Sibley et al., 2020; Richmond et al., 2022).

Lateralization and Handedness

Most people process input from the right side of the body in the left hemisphere of their brain and vice versa. As you may recall from 3.1 Physical Development in Infants and Toddlers this organizational characteristic of the brain is known as lateralization. The left hemisphere typically handles language-related tasks such as reading, speaking, and thinking, while the right hemisphere specializes in emotional expression, musical ability, and recognition of visual-spatial relationships used in geometry, art, and navigation.

Lateralization begins at birth. The hemispheres are independent, and they work together to process experiences and respond. For example, oral communication, a function of the left hemisphere, also relies on right-hemisphere functions such as visualization. Therefore, there is no such thing as being “left-brained” or “right-brained.”

Handedness is another example of lateralization in action. About 10 percent of children across cultures and continents prefer to use their left hand, a preference linked to differences in brain organization and genes. Whereas about 95 percent of right-handed people show typical lateralization patterns (like language centers in the left hemisphere), only about 75 percent of left-handers do (de Kovel et al., 2019). Though some studies indicate that most children develop an obvious hand preference by six months of age, others find that stable preferences may not appear until age nine or even later (Scharoun & Bryden, 2014). However, by that time, most children have already been required to favor one hand over another in school tasks like coloring, writing, and using a mouse.

Differences in cognition, artistic expression, and athletic prowess between left-handed and right-handed children are often reported. However, no scientific evidence suggests that these differences exist (McManus, 2019). Nonetheless, children who are left-hand dominant often struggle with products designed for right-handers, such as scissors, seat belts, and school desks. Though we can learn to use our non-dominant hand quite well, hand dominance appears to be highly heritable and unchangeable. Even identical twins, who share the same genes, may favor different hands; each has the same 10 percent chance of being left-handed. This indicates that handedness results from a combination of environmental and genetic influences (Schmitz et al., 2017).

Macronutrients

The body requires carbohydrates, proteins, and fats in large amounts. These groupings are commonly known as macronutrients. They play a vital role in the growth and development of preschool children.

Carbohydrates are the body’s primary source of energy. Preschool children need them to fuel their high level of activity and support their growing brains, and those who do not get enough may feel tired and weak. Good sources of carbohydrates include fresh fruits, vegetables, and whole grains. These foods also provide the fiber, vitamins, and minerals essential to maintain health.

Proteins are important for building and repairing tissues in the body. Preschool children need proteins for growth and development, and for maintaining a healthy immune system. Without sufficient protein, children are at higher risk for infections (Fan et al., 2022). Good sources of animal-based proteins include lean meats, fish, eggs, and dairy products such as cheese and yogurt. There are many good plant-based protein sources as well, like soy (for example, tofu), legumes like peas and beans, and nuts.

Fats are another important macronutrient preschool children need to maintain energy and growth, and to better absorb vitamins and minerals. Low intake of healthy fats can affect brain development and is associated with behavioral and learning problems (Chianese et al., 2018). Sources of healthy fats include nuts, seeds, avocados, and fatty fish like salmon. On the other hand, it’s important to limit saturated fat intake. Saturated fats are found in animal meat products and rich dairy, including butter, cream, and many cheeses.

Stricter government regulations have reduced the number of food items with trans fats, but these substances are still common in processed snacks like crackers and cookies, frozen desserts, and fried foods of all kinds. Trans fats and saturated fats are associated with an increased lifetime risk of heart disease and other health problems (Houston, 2018; Pipoyan et al., 2021).

Healthy Habits

Parents and caregivers often worry about how much their children are eating rather than considering what they are eating. Children are quite good at regulating their own nutritional habits if given a chance. Serving less nutritious food (like the children’s meals at many restaurants) because it is easier to get children to eat them generally leads to poor habits. Children who consume a lot of high-fat, sweet, and salty foods may fail to acquire a taste for other flavors. In contrast, adults who model positive eating habits, gently encourage children to try new foods, and avoid arguments over meals have a positive impact on children’s eating behavior (Figure 5.6). In short, children who are introduced to healthy vegetables and proteins on a regular basis will grow accustomed to them (Cardona Cano et al., 2015; Mahmood et al., 2021; Mazza et al., 2022; Nekitsing et al., 2018).

Figure 5.6 When a variety of healthy foods is available, children are more likely to enjoy these options as much as their parents and caregivers do. (credit: modification of work “Family eating a meal (2)” by Rhoda Baer, National Cancer Institute (NCI)/Wikimedia Commons, Public Domain)

A significant proportion of U.S. children and families live in a food desert, a geographic area where fresh food is not readily available (Figure 5.7). These usually lower-income, inner-city or rural areas spark a dependence on unhealthy fast food or frozen prepared food. Residents may therefore rely on the limited offerings of nearby convenience stores or travel long distances for their meals, which can be costly and incur further financial strain. Limited access to affordable and nutritious food can lead to a variety of health problems, including obesity, diabetes, and heart disease (Ziso et al., 2022).

Figure 5.7 This 2015 map shows the location of food deserts in the United States. The colors indicate the percentage of the population in the area with low access to or proximity to grocery stores. (credit: modification of work “Population, Low Access to Store (%), 2015” by USDA Economic Research Service, U.S. Department of Agriculture/ERS, Public Domain)

Efforts to increase access to healthy foods include tax incentives to build supermarkets, host farmer's markets, and develop community gardens in underserved areas. Addressing food deserts is an important step in promoting food equity and improving public health outcomes, particularly for vulnerable populations like those who are impoverished or have food allergies that further restrict food choices.

Sleep Duration

According to the WHO, preschool children typically need ten to thirteen hours of sleep every twenty-four hours and to maintain consistent sleep and wake times. Children who have stopped napping sleep more at night, getting the same amount of sleep overall as their peers who nap (Ward et al., 2008; WHO, 2019).

Though evidence is limited, studies indicate that getting fewer than ten hours of sleep per night during early childhood is associated with a higher risk of accidents and hypertension, and a lower quality of life later. However, it’s difficult to know whether those associations are due to poor sleep habits or to family and environmental variables, like a household that is less consistent about children’s sleep habits, nutrition, and activity levels. A combination of factors may be responsible for poorer outcomes among children who sleep less than ten hours per night (Paruthi et al., 2016; Spruyt, 2019). Substantial evidence associates less-than-optimal sleep with unhealthy weight gain and obesity. These outcomes are likely due to changes in appetite, including excessive snacking and increased calorie consumption, and the way the body metabolizes energy (Papatriantafyllou et al., 2022).

Sleep Problems

Sleep problems are not uncommon in young children and can manifest in many ways. Fear of the dark and of being left alone can make it difficult to fall asleep or cause children to wake during the night. Parenting blogs and social media sites commonly report that up to half of children aged three to six have bad dreams at least once per week, but the prevalence is probably much lower, perhaps as low as 13 percent (Simard et al, 2008).

Between 1 percent and 6.5 percent of children experience night terrors (or sleep terrors), which appear to peak between ages five and seven. Unlike dreams, which occur during REM sleep, night terrors occur during deep sleep and initiate intense fear or terror. Children may suddenly sit up, scream and thrash, and appear to be in a state of panic or terror, sweating and having a rapid heart rate. In this state, they aren’t fully awake and aren’t easily comforted. They may not even respond to their parents or caregivers (Leung et al., 2020).

While the exact cause of night terrors is unknown, treatment usually consists of reassuring the child and keeping them safe during an episode. Children should not be forced to wake up. However, it is often effective to briefly wake them thirty minutes before the time terrors usually begin, typically during the first third of the night, and determine whether that intervention makes a difference. Medication and other treatments are not generally indicated, and most children grow out of night terrors by early adolescence (Leung et al., 2020; Van Horn & Street, 2022). Most also have no memory of the night terror when they awake.

Vaccination

The use of childhood vaccines in countries with high rates of national immunization coverage is responsible for the near-elimination of serious diseases such as polio and measles. Because there are no cures for these ailments, vaccinations are the best defense against severe illness and mortality (Figure 5.9).

Figure 5.9 Adhering to recommended vaccination schedules provides an easy way of improving children’s overall health outcomes. (credit: “First under 5 COVID vaccine at NHC Pax River 22-0001-480 (7334629)” by Navy Medicine/Flickr, Public Domain)

Vaccines commonly administered in the United States protect against measles, mumps, rubella, hepatitis B, polio, diphtheria, tetanus, and pertussis (whooping cough). Although children continue to get vaccines until their second birthday, after which they receive mostly booster shots, there is no evidence they overburden the immune system (Glanz et al., 2018; Gregson & Edelman, 2003; Nicoli & Appay, 2017), as some people fear. Many public schools require vaccinations for entry, and the CDC and other health organizations provide recommended vaccination schedules as well as catch-up schedules if a child has fallen behind or otherwise needs an alternative schedule.

Another unfounded concern, that vaccinations are linked to the occurrence of developmental disorders, became popular in 1998, when British physician Andrew Wakefield and his colleagues published a now-retracted study in the scientific journal The Lancet. Without proper evidence, Wakefield suggested the MMR (measles, mumps, rubella) vaccine predisposed children to autism (as the disorder was then called) (Wakefield et al., 1998). The report received wide media attention despite its limitations, including having a sample size of only twelve, a poor design, and merely speculative conclusions. As a result, MMR vaccination rates declined significantly (Motta & Stecula, 2021; Rao & Andrade, 2011). The British Medical Journal later published a series of articles outlining fraudulent behavior among the research team and accusing Wakefield of knowingly publishing false information (Godlee et al., 2011; Kmietowicz, 2010). Wakefield was stripped of his medical license for misconduct, dishonesty, and unethical behavior. Nevertheless, his baseless influence affects vaccination rates decades later as widespread distrust of vaccines remains, despite their proven safety (Béres et al., 2023; Nuwarda et al., 2022).

Globally, immunization among children under five years old decreased from 86 percent in 2000 to 83 percent in 2023. Although the measles vaccine is widely available and inexpensive, limited distribution still exists in countries with low per-capita incomes or weak or disrupted health-care infrastructures; in 2022 136,000 deaths were attributed to measles, primarily among children under age five (WHO, 2024). Nevertheless, this vaccine has reduced the global death rate of measles by more than 83 percent since widespread use began (WHO, 2023). In the United States, the measles vaccine gained widespread distribution beginning in the 1960s, contributing to a decline in cases from about 750,000 a year to fewer than 200 annually by 2024 (CDC, 2024). Probably due to persistent misinformation, measles cases rose as high as 1,274 in 2019 before dropping again during the COVID-19 pandemic when children were isolated from one another. The WHO has recently rolled out its late-stage trial of a malaria vaccine (UNICEF, 2023) to combat one of the leading causes of childhood deaths globally.

Illness, Unintentional Injury, and Mortality

Globally, 15 percent of deaths among children under five are caused by diarrheal diseases. Unsanitary conditions and lack of access to safe drinking water are contributing factors. Often, simply drinking liquids to avoid dehydration will prevent death. However, of children who contract diarrheal diseases, fewer than half have access to clean drinking water (UNICEF, 2023). Along with diarrheal diseases, malnutrition and malaria remain leading causes of death in children under five, though they are virtually non-existent in wealthy countries. Many high-income countries have seen a strong decline in childhood death from diarrheal disease, largely thanks to better prevention science and rapid treatment (Santosham et al., 2019). In the United States, deaths from diarrheal diseases accounted for fewer than 0.1 percent of deaths in children younger than five (CDC, 2022).

After age one, unintentional injuries—accidents—become the leading cause of premature death in childhood. Despite continued improvements in safety standards for motor vehicles and consumer products, the overall decline in accidental deaths has stagnated. There are also persistent racial and ethnic differences in accident rates (CDC, 2022), almost certainly due to socioeconomic disparities between minorities and those who are more advantaged. Socioeconomic status (SES) plays a role in injuries related to fires, drownings, and falls, likely due to limited financial resources and education, less supervision, and lack of safe play areas. Low SES is also linked to higher rates of accidents and deaths related to burns, fires, drownings, and poisonings outside the United States (Mahboob et al., 2021; Moshiro et al., 2021; Tyler et al., 2017).

Ensuring safety and preventing homicides in both elementary and secondary schools has become an increasingly pressing matter in the United States. Homicide is currently one of the leading causes of death for children across age ranges—the fourth leading cause of death for children ages one to four, the fifth leading cause of death for children in elementary and middle school, and the third leading cause of death for high-school aged teens (National Center for Health Statistics, 2021). Between January 2009 and May 2018, there were 389 reported school shootings in the United States, compared to fewer than 40 in all other countries combined (CNN, 2018; Cox et al., 2024). Despite this alarming trend, there have been no widespread efforts to restrict the availability of firearms since the Columbine High School shooting in 1999. Many schools have implemented emergency plans and procedures for responding to threats, increased the use of physical barriers and security measures, and established partnerships with law enforcement and mental health providers.

Dangers to children and adolescents in other parts of the world are very different. In some regions of Africa and Asia, such as Yemen and the Central African Republic, where political instability remains pervasive, military regimes abduct and enslave children for use in armed conflict (Human Rights Watch, 2023; United Nations [UN], 2023). In other countries, such as South Sudan and Afghanistan, both government and opposition forces use residential areas, schools, and students as shields, posing significant threats and making children prime targets of violence. Some progress in protecting children has been made by the UN's "Children, not Soldiers" campaign launched in 2014 (UN, 2023).