3.1: Physical Development in Infants and Toddlers

Learning Objectives

By the end of this section, you will be able to:

• Identify patterns of physical growth in infants and toddlers
• Describe sleep patterns and safe practices for putting a young child to sleep
• Describe eating patterns and factors that influence eating preferences
• Describe typical brain growth and maturation in infancy and toddlerhood
• Identify health risks in the first two years of life

Alexander weighed almost 10 pounds at birth. That put him in the top fifth percentile for weight, meaning he was bigger than 95 percent of newborn babies. Today at his eighteen-month checkup, his pediatrician comments that Alexander is now in the fiftieth percentile for his weight. Alexander’s adoptive parents ask whether they should be concerned that he has dropped to a lower percentile, but the pediatrician explains that many factors influence growth during the first few years of life, including multiple genes and environmental experiences. By toddlerhood, all these factors together will move most children into a percentile that will likely stabilize in later childhood.

The first two years of life are a time of incredible change. The earliest months can be a haze for many new parents and caregivers, who may be feeling sleep deprived and struggling to learn what their baby needs or wants. But a few months later, a baby who used to wake hourly may sleep for longer stretches. Caregivers find joy in the baby’s evolving smiles and coos. And two years later, that toddler may well tell grownups what they want or don’t want by saying a few words or running toward a favorite toy or turning away from a disliked food.

There are many typical changes in the first two years of life, starting with patterns of growth and maturation in weight, height, and head size. Additional considerations include sleep and feeding, important aspects of health and development at any age, and moving on to brain growth and health risks. Moreover, cultural, social, and other experiences may influence maturation in these areas.

Patterns of Growth and Maturation

Growth in utero and the first two years of life follows three predictable patterns. Recall that in the cephalocaudal pattern, growth starts with the head and moves downward toward the torso and then the legs and toes Figure 3.2. For example, the head and upper body will grow before the rest of the body. In the proximodistal pattern, development begins in the center of the body and moves outward. That is, before birth the internal organs develop before the arms and legs. After birth, the infant’s torso grows before the arms and legs, followed by the toes and fingers. In addition to growth, children will usually develop control over the center of their body first and then gradually gain control over the parts of their body further from their middle such as their hands and feet. This is why you typically see infants reaching for objects before they can successfully grasp and manipulate toys.

Figure 3.2 Proximodistal growth occurs at the center of the body before the extremities, whereas cephalocaudal growth begins at the head of the body before moving to the lower body. (CC BY 4.0; Rice University & OpenStax)

The third pattern of growth is the mass-to-specific pattern, which begins with gross motor skills (controlled by large muscle groups) and moves to fine motor skills (controlled by smaller muscle groups). During infancy, children make tremendous gains in gross motor skills, such as rolling over and sitting, before they focus on fine motor skills such as grasping objects and feeding themselves.

Other dramatic physical changes occur after birth as well. For example, newborns may temporarily have an irregular cone-shaped head due to the birthing process, and/or downy hair (lanugo) on the shoulders and back that typically disappears in the first weeks or months. By around eleven weeks, newborns have gained enough weight to help them regulate their body temperature (Joseph et al., 2015). Teething occurs during the first year as well, with a first tooth typically emerging between four and six months (but sometimes after the first birthday). The bottom front teeth usually develop first Figure 3.3, followed by the top front teeth, and most children will have all their teeth by the time they’re two or three years old.

Figure 3.3 Signs of teething in infants include sore gums, flushed cheeks, rubbing of ears, more than usual dribbling, and poor sleep. (credit: modification of work “Baby aaron” by Philippe Put/Flickr, CC BY 2.0)

Among all these milestones, professionals usually monitor a few specific changes: weight, height, and head circumference.

Weight, Height, and Head Circumference

Average birth weight in the United States is about 7.5 pounds, and average length is 19.5 inches (Centers for Disease Control and Prevention [CDC], 2010), but average birth weight varies globally. For example, in Africa it is 6.9 pounds (3,149 grams), followed by Central America (2,874 grams or 6.33 pounds) and Asia (2,713 grams or 5.98 pounds) (Marete et al., 2020). In the United States, newborns weighing less than 5.5 pounds are considered low birth weight, which can be a risk factor for developmental problems. Rates of low birth weight also vary by region. Approximately 10 percent of U.S. babies are low or very low birth weight (less than 3 pounds 4 ounces) (Osterman, 2024). The rate is lower in Africa (3.9 percent) but higher in both Central America (15.6 percent) and Asia (20.2 percent) (Marete et al., 2020). Many babies with low birth weight are able to catch up on their growth and weight gain over the first couple of years (Vizzari et al., 2023).

During their first week, newborns lose about 5 to 8 percent of their birth weight as they adjust to life outside the womb, but they typically regain it by the end of the second week (Graber, 2023). From then on, they generally gain 4 to 8 ounces per week and can grow 1.5 to 2 inches in their first month (Graber, 2023). Birth weight typically doubles by five months, and the average twelve-month-old weighs 22 pounds (Graber, 2023). After the first year, growth slows, and children gain about 5 pounds in the next year. Meanwhile, infants grow about 1 inch per month during the first year, and another 4 to 5 inches by age two years (Graber, 2023) (Figure 3.4).

Figure 3.4 Look across these two sets of pictures of the same child growing from infancy to toddlerhood. Some common milestones across this period include raising the upper body, standing while holding onto an object for support, increasing development of fine motor skills, and growth in gross motor skills. What changes are apparent? (credit “6 months” top: “Child crawling on play mat” by Kerry Ceszyk/Flickr, CC BY 4.0; credit “12 months” top: “Child outside” by Kerry Ceszyk/Flickr, CC BY 4.0; credit “18 months” top: “Child dyeing eggs” by Kerry Ceszyk/Flickr, CC BY 4.0; credit “24 months” top: “Child with snowman” by Kerry Ceszyk/Flickr, CC BY 4.0; credit "6 months" bottom: "Child sitting" by Sarah Evans/Flickr, CC BY 4.0; credit "12 months" bottom: "Child with soccer ball" by Sarah Evans/Flickr, CC BY 4.0; credit "18 months" bottom: "Child in swing" by Sarah Evans/Flickr, CC BY 4.0; credit "24 months" bottom: "Child playing with blocks" by Sarah Evans/Flickr, CC BY 4.0)

Monitoring head circumference is crucial for the early detection of cognitive problems during early childhood, and this measure is positively correlated with cognitive functioning in children and adults (Gale et al., 2006). In a child’s first year, the skull grows about 0.4 inches (1 cm) per month. At birth, the head is disproportionately large, about one-quarter of total body length. In adulthood, the head is approximately one-eighth of total body length. The proportion of the head to the body is part of the reason it’s hard for a newborn (or even a two-month-old) to raise their head.

Growth as an Indicator of Healthy Development

Physical growth is influenced by culture, environment, and genetics and their interaction, including epigenetic processes occurring both prenatally and after birth (Getaneh et al., 2021; Koemel & Skilton, 2022; Muglia et al., 2022). Nutrition in utero and in early life affects physical growth and individual health and may be related to health issues such as allergies (Acevedo et al., 2021).

Pediatric health providers typically measure the child’s height and weight during routine wellness visits and enter them in a growth chart. Growth charts are percentile curves that track how the child is developing according to their age. The World Health Organization’s (WHO) growth charts are internationally standardized, and separate versions exist for boys and girls Figure 3.5. The CDC recommends the WHO Growth Charts be used for children ages two years and younger (CDC, 2010).

Figure 3.5 This is the WHO chart for girls from birth to age twenty-four months. What is the typical length (height) and weight for a twelve-month-old? (credit: modification of work “Birth to 24 months: Girls Length-for-age and Weight-for-age percentiles” by WHO Child Growth Standards/Centers for Disease Control and Prevention, Public Domain)

Link to Learning

Growth charts are used to track changes in body measurements in children. They are useful for seeing trends in growth over time as well as for identifying any concerns about a child’s rate of growth. The WHO Growth Charts lets you view charts for a variety of growth measures for boys and for girls between birth and 2 years of age.

Growth charts measure height/length for age, weight for age, and height-to-weight ratio. By showing trends over time, they can help identify any growth problems. For example, if a child typically in the fortieth percentile for weight drops to the twentieth percentile in one year, the doctor will look for possible reasons. Growth charts are also used to identify failure to thrive, a condition in which growth is significantly below the norm for the child’s age. The most common cause of failure to thrive is inadequate caloric intake, but other risk factors include medical conditions and environmental factors such as food insecurity or abuse or neglect (Goodwin et al., 2023). Failure to thrive can result in developmental delays if not resolved (Hussein et al., 2023).

Irregular growth patterns could indicate health or nutrition problems and a need for intervention. However, a child’s gestational age, birth weight, and parents’ stature also influence growth patterns and account for individual differences, as do some cultural factors. For example, in Papua New Guinea, breastfeeding is universal and continues until children are five years of age. In addition, the typical diet is low in protein. This pattern of nutrition is believed to be the reason some individuals in this region tend to be smaller overall and to grow slowly (Jenkins et al., 1984).

Sleep and Variations

Children’s overall health is also influenced by their sleep. Sleep is a biopsychosocial process, meaning it is influenced by biological, psychological, and social factors. It is essential for infant brain development, immune support, and growth (Liu et al., 2023), and involve various sleep patterns, practices, and safety measures.

Development of Sleep Patterns

At two years old, children will have spent about 33 percent more time asleep than awake. What happens while young children sleep? To study sleep, researchers use an electroencephalogram (EEG). These are images of brain oscillations or waves created by the electrical activity of neurons and captured by a cap fitted with electrodes. The recorded height and speed of the wave indicate the type of sleep occurring.

There are two types of sleep. REM is rapid eye movement sleep, sometimes called active sleep because the eyes move while closed, and EEG waves in this sleep are similar to EEG waves in waking. REM sleep includes dreaming, memory consolidation, emotional processing, and brain development. NREM or non–rapid eye movement sleep is divided into three stages: stage 1 is the lightest sleep and stage 3 the deepest. NREM sleep consists of deeper levels of relaxation including slowed heart rate and respiration. A complete sleep cycle includes REM and NREM sleep and takes about ninety minutes for adults and children older than five years. Around two to three years of age, however, the sleep cycle is sixty minutes.

REM sleep occurs more often and lasts longer for infants than for preschoolers, but preschoolers still get more REM sleep than adults. Approximately 50 percent of sleep is REM sleep up until the first year, and then NREM sleep increases until adulthood, when REM sleep governs about 20 to 25 percent of our sleep. We experience more variation in sleeping patterns during our first year than at any other point in our lives (Table 3.1). Compare these recommendations: a newborn should sleep fourteen to seventeen hours a day, and a two-year-old should sleep eleven to fourteen hours, whereas an adult should sleep between seven and nine hours (Mireku & Rodriquez, 2021).

Table 3.1 Typical Sleep Patterns for Newborn to Two Years Old

Age	Nighttime Sleep	Daytime Sleep	Total Sleep
Newborn	8–9 hours	8 hours	16 hours
3 months	9–10 hours	4–5 hours	15 hours
6 months	10 hours	4 hours	14 hours
9 months	11 hours	3 hours	14 hours
1 year	11 hours	3 hours	14 hours
1.5 years	11 hours	2.5 hours	13.5 hours
2 years	11 hours	2 hours	13 hours

Newborns can’t distinguish night from day and require multiple nighttime feedings, but they will start to sleep less during the day by three months. At around six months of age, infants typically begin waking less often at night. By twelve months of age, most infant sleep occurs at night (Barry, 2020), but night waking can continue through the first two years of life (Paavonen et al., 2020). Fragmented sleep can be considered a sleep problem, particularly in some Western cultures where night waking and bed sharing are less typical (Barry, 2021).¹ As a result, many guidelines and recommendations about infant sleep in these cultures emphasize the need for solitary and continuous sleep.

Infant Sleep Assistance

Infants offer cues that they are ready to sleep by crying, yawning, rubbing their eyes, or being fussy (when overtired). But this doesn’t always mean they get to sleep quickly or easily. In some Western societies, bedtime strategies include physical comforting (cuddling), movement (rocking, car rides), social comforting (songs, stories), and passive comforting (standing or sitting nearby). In rural Guatemala as well as in many other areas, swaddling (tightly wrapping the infant in fabric during sleep for its calming effect) is a common practice (Oh et al., 2024) (Figure 3.6). Mayan mothers typically hold their children but do not use stories or lullabies. Another strategy that has been shown to promote sleep in crying infants is brief (five-minute) carrying by caregivers (Ohmura et al., 2022).

Figure 3.6 A child swaddled in a blanket for sleep. Some blankets are made for this purpose. (credit: “Swaddled infant” by Joanne Lee/Flickr, CC BY 4.0)

Some Anglo European cultures, such as the United States and United Kingdom, tend to favor infant autonomy for getting to sleep (Giannott & Cortesi, 2009),² meaning the baby is encouraged to fall asleep on their own, perhaps with self-soothing mechanisms like a soft toy or a pacifier. Another option is sleep training, which is sometimes suggested for infants over six months of age (Reuter et al., 2020). One sleep training method is the Ferber (or “cry it out”) method, in which parents leave the child alone for a set time to help develop the ability to fall asleep independently, even if the baby cries. Another sleep training option is a soothing bedtime routine, which creates positive associations with bedtime.

Many activities in Table 3.2 can form part of a bedtime routine. A caregiver might give the baby a bath, read a story, turn down the lights, put on some quiet music while rocking the baby, and then place the baby in bed. A few strategies, such as rocking and feeding, can sometimes reduce the baby’s ability to fall asleep independently because babies may become reliant on these external cues from caregivers to sleep and thus may not develop their own self-soothing strategies. Sleep researchers recommend putting the baby down when they are relaxed but before they fall asleep and soothing them rather than picking them up again.

Table 3.2 Bedtime Routines

Bedtime Activity	Description
Swaddling	Wrapping infant in a blanket to reduce movement; should typically be stopped when the infant can turn over
Rocking	Holding the baby while swaying, or rocking them in a cradle
Bathing	Giving a bath before bed, perhaps adding a gentle massage
Reading	Reading a simple story
Singing	Singing a lullaby or other quiet song
Darkening the room	Signaling the brain to release sleep hormone as well as reducing the possibilities of environmental stimulation
Playing a sound machine	Using only enough volume to mask other noises
Feeding	Breastfeeding or bottle feeding until the baby is asleep

Research has demonstrated the efficacy of behavioral interventions and sleep health education in treating sleep problems during infancy and early childhood (Liu et al., 2023). Behavioral interventions often focus on the parents’ behavior, training them to allow the child to self-soothe, such as by waiting a few minutes before comforting a crying six-month-old. These interventions are associated with improved night sleep for both the parent and infant, and reduced symptoms of maternal depression (Liu et al., 2023).

Infant Sleeping Arrangements and Safety

Infant sleeping arrangements, where the infant sleeps and with whom, can be a controversial topic and one deeply influenced by familial and cultural factors. Infants typically sleep separately from parents in the United States, United Kingdom, and Australia, though that can include sleeping in a crib or bassinet in the same room. However, even within these countries, variations can range from having a separate room for an infant to co-sleeping, in which the infant and caregivers share the same bed. Globally, co-sleeping is a common practice (Barry, 2017) adopted by as many as 70 percent of families (Huang et al., 2010). In the United States, the rate of regular or occasional co-sleeping has risen to 50 percent over the past two decades, suggesting a possible cultural shift (Austin et al., 2017).

No matter which sleep methods and arrangements caregivers choose, they need to consider the infant’s safety. A sudden unexpected infant death (SUID) occurs when an infant under one year of age dies unexpectedly, often while asleep (Figure 3.7). SUID occurs through accidental suffocation, unknown causes, and sudden infant death syndrome (SIDS) (Sidebotham et al., 2018). SIDS is deemed the cause when the death occurs during sleep and evidence does not indicate a specific single cause. Some research studies associate SIDS with the body’s failing to regulate physical functions, like processing oxygen and carbon dioxide (Kim & Pearson-Shaver, 2023). SIDS has been a leading cause of death in infancy in many countries for decades (Sidebotham et al., 2018).

Figure 3.7 Sudden unexpected infant deaths (SUIDS) include deaths attributed to sudden infant death syndrome as well as to suffocation and strangulation deaths. However, almost one-third of SUIDs occur from no known cause. (data source: CDC/NCHS; attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

In 1994, Back to Sleep and similar campaigns were introduced in the United States, several European countries, and New Zealand to reduce infant deaths related to sleep. These efforts recommended back or side sleeping for newborn infants. For example, sleeping on their backs reduces infants’ risk of choking or aspiration (Michigan.gov, n.d.; Moon & Task Force on Sudden Infant Death Syndrome, 2011). Similar campaigns were introduced in other countries, and rates of SIDS decreased between 42 and 92 percent (Sidebotham et al., 2018).

In the United States, the American Academy of Pediatrics (AAP) currently recommends the A-B-C method, which prioritizes the infant sleeping alone (A), on their back (B), and in a crib (C) (Moon et al., 2024). Sleeping alone in a bed that is free of soft objects, toys, and blankets reduces infants’ suffocation risk. Infants should always be put to sleep on their backs to reduce choking risk. As they get older, they will learn to roll over and move around. Infants should sleep in a safety-approved crib or bed rather than slings, high chairs, or other furniture in order to remain flat, which further reduces risks of suffocation and choking. Use of these methods has reduced the incidence of sleep-related deaths (Shapiro-Mendoza et al., 2023). The AAP also recommends room sharing for the first six months to reduce the risk of SIDS (Moon et al., 2022).

However, many parents find these recommendations challenging, particularly those related to co-sleeping (Moon et al., 2024). Co-sleeping has both risks and benefits. Some studies have associated it with an increased risk of SIDS, specifically when the primary caregiver has used substances such as cigarettes or alcohol (Bains & Mittal, 2023).³ Co-sleeping may also increase sleep problems in toddlers (Yang et al., 2022). However, the benefits can include more sleep for both the caregiver and baby, as well as easier breastfeeding at night and increased duration of breastfeeding (Salm Ward, 2015). Sleep choices may vary based on each family’s needs, lifestyle, and culture. For example, caregivers who breastfeed and do not drink heavily or smoke may find co-sleeping beneficial.

Link to Learning

Red Nose Australia outlines six recommendations for safe sleep and co-sleeping in this article.

Feeding and Variations

Like sleep practices, infant feeding reveals different cultural practices and social preferences. Should infants be bottle fed, breastfed, or both? Should they receive human milk, formula, or a combination? When should they get some solid foods? What types of solids should older infants have?

Human Milk and Formula

Caregivers may choose to feed their infant human milk, infant formula, or a combination of the two during the first six months of life. Each of these nutrient sources may be delivered via bottle feeding, breastfeeding, or chestfeeding (using a feeding tube attached to the nipple) (Bartick et al., 2021). Human milk may come from the birth mother or be donated (Bartick et al., 2021). The WHO, the AAP, and UNICEF recommend starting breastfeeding within one hour of birth and breastfeeding exclusively for the first six months of life (Meek et al., 2020). Human milk is associated with a lowered risk of SIDS (Moon et al., 2022) and provides nutrients and antibodies that reduce infants’ risk of respiratory tract infections, diabetes, colds and flu, and middle-ear infections (Meek et al., 2020). It also protects against gastrointestinal infections, particularly critical in developing countries where the water often isn’t safe for mixing with infant formula. In these countries, the risk of infant death from diarrhea and other infections is reduced when infants are exclusively breastfed (WHO, 2023).

In some cases, however, such as when a mother has certain health conditions or is taking certain medications that get passed through human milk, breastfeeding may not be advised. In addition, some parents may be unable to breastfeed due to work and family arrangements, physical ability, or adoption. Some may choose not to breastfeed based on individual preferences or needs. Finally, sometimes supplementing with formula is recommended, such as when the baby is failing to gain weight well or the nursing parent struggles to supply milk. Table 3.3 shows some of the benefits of human milk and formula feeding.

Table 3.3 Benefits of Human Milk and Formula (sources: Feldman-Winter et al., 2022; Martin et al., 2016; Fan et al., 2023)

Human Milk	Formula
Digestion: Human milk contains balanced nutrients that are easily digested by young infants.	Timing: Formula takes longer to digest, so infants can go longer between feedings and sleep for longer durations.
Immunity: Human milk contains immunoglobulins that provide immunity to many diseases.	Caregiver’s diet: Caregiver doesn’t need to worry about what they are eating or drinking, including alcohol.
Financial cost: Human milk may be less expensive or have no cost, though pumping supplies do entail a cost.	Options: Formula comes in a range of options, such as for infants with allergies or infants needing more calories for weight gain.
Convenience: Human milk, when breastfed, is the correct temperature, requires no extra equipment, and can be delivered immediately when nursing parent and baby are together.	Convenience: Anyone can feed the baby, which can distribute the caregiving load. Formula, when bottle fed, can also be delivered without privacy concerns in public settings.
Attachment: Human milk provided by breastfeeding releases oxytocin that promotes bonding.	Attachment: Any primary caregiver can use a bottle- or chestfeeding technique with formula, which can promote bonding for all caregivers.
Maternal health: Breastfeeding has been associated with a reduced risk of breast and ovarian cancers.	Maternal comfort or lifestyle: Formula provides a healthy option for those who cannot or prefer not to breastfeed or provide human milk.

Infant feeding practices also vary around the world. For example, breastfeeding rates tend to be lower in high-income countries (UNICEF & WHO, 2018), which may reflect differences in the way it is viewed and how likely a mother is to return to work while her child is still of breastfeeding age (Gallegos et al., 2020). Many caregivers choose a combination of human milk and formula as well as a combination of breastfeeding and bottle feeding.

Frequency of Feeding

As infants get older, the frequency of their feedings will change. The Dietary Guidelines for Americans and the AAP recommend that infants are only fed human milk and/or formula until six months of age (CDC, 2023a). The stomach of a newborn infant is small and easily filled. As a result, they may eat as often as every one to three hours. Over the first several months, the time between feedings will increase, but feedings may still be needed every two to four hours. By nine to twelve months, when the stomach has increased in size, infants are able to eat more at one time and often need only three feedings a day.

A common recommendation is to follow the infant’s hunger cues (sucking noises, open mouth, hands near mouth) and fullness cues (turns head away, spits out, is distracted) (Samour & King, 2005). Such responsive feeding can help develop an infant’s sense of trust and security as they learn that their caregivers will provide what they need. Caregivers should feed infants slowly and patiently, encouraging but not forcing them. They can also maintain eye contact while talking to the infant throughout the feeding (WHO, 2023).

Complementary Feeding

The WHO, AAP, and UNICEF also recommend introducing nutritious and safe solid foods at six months of age. However, the addition of solids is considered complementary feeding, because the primary source of nutrition at this stage should still be human milk and/or formula.

The time to start solid foods is based on the infant’s ability to sit up, support their head, and push up with straight elbows from a lying-down position (Figure 3.8). Babies will also show interest in food by leaning forward and opening their mouth. Introducing solid food before six months may interfere with the child’s getting enough nutrients from milk, though culture does play a role in determining when parents introduce solids. For example, many French mothers introduce solids at around four to five months (Courtois et al., 2021).

Figure 3.8 These are signs that a baby is ready to start eating solid food. (credit left: modification of work “20210805-FNS-UNC-0003” by U.S. Department of Agriculture/Flickr, Public Domain; attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

At six months, infants can be introduced to cereals and pureed fruits and vegetables. To monitor possible allergic reactions to foods, safe foods should be introduced one at a time, for several days each. Even at this age it is important to ensure that an infant’s diet contains a balance of macronutrients, with about 50 to 55 percent of daily calories coming from carbohydrates, about 20 percent from proteins, and about 20 to 25 percent from fats (Faizan & Rouster, 2024). A healthy infant should eat about 1,000 calories per day (Patel & Rouster, 2023). The transition from liquid to solid foods is crucial for the development of oral motor skills and oral processing abilities, particularly during the first year of life (Nicklaus et al., 2015).

Infants are ready to progress to finger foods like infant puffs (small snacks that dissolve easily) and dissolvable crackers when they can sit independently, grasp and release food, and chew it (even without teeth) (Kleinman & Greer, 2020). As the child develops more motor skills and socialization, they will be able to eat most of the same foods as the rest of the family and start drinking from a cup and using a spoon.

Factors in Food Preferences

Eating behaviors can vary based on infants’ sensory sensitivity, temperament, exposure to different foods, and caregivers’ feeding practices. Infants prefer sweet solutions to water at birth; by six months of age, however, this continued preference was associated with dietary exposure to sweet foods (Forestell & Menella, 2017). Therefore, experts recommend offering complementary foods without added sugars and salt. This helps to set a lower threshold for sweet and salty tastes later in life (De Cosmi et al., 2017). Infants are also born with a natural tendency to reject bitter foods, but recent research has shown that adding a small amount of sugar or salt increases the likelihood that infants will accept more novel, slightly bitter foods such as kale (Johnson et al., 2021). Nursing mothers usually eat a variety of foods, which may explain why breastfed children tend to try new foods and become less picky eaters than formula-fed infants (De Cosmi et al., 2017). Regardless of whether an infant is human milk or formula fed, they will also have gained some experience with flavors through tasting the amniotic fluid in the womb (De Cosmi et al., 2017).

Food acceptance is closely related to sensory processing, because foods differ in taste, smell, appearance, texture, and even sound (Blissett & Fogel, 2013). Exposure to a variety of foods during the complementary feeding period is related to greater acceptance of new foods (De Cosmi et al., 2017). Children who are not exposed to a variety of foods may become unwilling to try new tastes. When new foods are introduced within a positive social environment and parents model healthful eating behaviors, children may be more willing to try foods. For example, if parents frequently eat quinoa and carrots, the infant may be more likely to eat quinoa and carrots due to modeling and exposure. It takes six to fifteen exposures to a new food before we can identify a child’s preferences and they begin to eat more of it (De Cosmi et al., 2017). Thus, in order to set up lifelong healthy eating patterns, parents should frequently and patiently offer healthy food choices.

Brain Growth and Development

If you’re having a bad day, do yourself a favor and search online for “newborn giraffes.” You’ll be greeted with funny and adorable footage of baby giraffes awkwardly standing up and trying to take their first steps mere hours after birth. What takes less than a day for a giraffe takes a year for human children, however. Why is this?

Many of our inborn motor skills are reflexes. A reflex is an involuntary movement that originates in our brainstem in response to stimulation. Reflexes are important for brain development, and most reflexes will develop into controlled motor skills, like grasping by four months and walking by twelve months. These motor and cognitive changes start with neurons, our body’s messengers, and relevant aspects of brain development.

Neural Growth

The brain, a 3-mm neural tube the size of a grain of rice during the embryonic stage, is estimated to have 100 billion neurons at birth (ten Donkelaar et al., 2023). A neuron (Figure 3.9), a specialized nerve cell, is the body’s messenger and consists of three parts:

• cell body (soma): manages the messaging, keeps neuron functioning
• axon: carries the message away from the cell body, contains terminal buttons at the end to help release the message, and is covered with a fatty white substance called myelin that helps transmit messages faster and more efficiently
• dendrite: receives the message and transmits it to the cell body

Figure 3.9 Each part of the neuron plays a role in communication between the brain and the body. (CC BY 4.0; Rice University & OpenStax)

The development of new neurons, termed neurogenesis, begins prenatally. Once neurons have formed, they begin to form connections with existing neurons. Neurons are the only cells in the body that aren’t directly connected to each other but instead are separated by a gap or synapse (Figure 3.10). Thus, the process of neurons connecting is called synaptogenesis. Neurons release neurotransmitters, the chemical messengers of the nervous system, into the synapse from the axon terminal, where dendrites of the next neuron receive them. By age two years, children have approximately 15,000 synapses or neural connections per neuron (Shonkoff & Phillips, 2000). These connections grow stronger if they’re used frequently. If not, they die off in a process called synaptic pruning. Think of synaptic pruning as the brain’s way of removing the clutter of unused synapses. As a result of overproduction of synapses during the first two years, toddlers may have twice as many synapses in their cerebrum/cerebral cortex as adults, so pruning continues into the teen years (Johnston, 2004). Beginning prenatally, myelination also occurs, the process by which the axon develops its myelin sheath. Myelination occurs primarily in the first two years of life but is still present throughout the lifespan.

Figure 3.10 The gaps between neurons are called synapses. (CC BY 4.0; Rice University & OpenStax)

By the age of two years, a child’s brain structure typically has the appearance of an adult brain (Johnson, 2001). The cerebral cortex/cerebrum is composed of multiple layers of gray matter, the outer layer of the brain, made up of the cell bodies of the neurons, particularly in the cerebrum/cerebral cortex and the cerebellum. Gray matter also exists in the brain stem and the cord. The interior is white matter that is composed of many bundles of axons that connect neurons to different regions with functional circuits/pathways.

The most rapid increase of white matter in the brain happens during the first two years (Johnson, 2001) as axons become increasingly myelinated. White matter is crucial for the efficient transmission of information throughout the brain and occurs along with cognitive and behavioral development (Stephens et al., 2020). Around age eight months to one year, the white matter associated with the frontal, parietal, and occipital lobes becomes visible on magnetic resonance imaging (Stephens et al., 2020).

Link to Learning

It is through experiences that neurons form synapses with nearby neurons, a process critical for brain development. How experiences shape the development of a child’s brain is described in this video on connections and brain development from Harvard University.

Brain Specialization

The cerebral cortex, also known as the cerebrum, is one of the areas of the brain that controls voluntary behavior and thought (Figure 3.11). It has two hemispheres connected by the corpus callosum, and each hemisphere has four specialized lobes:

• The frontal lobe controls thinking, planning, memory, and judgment.
• The parietal lobe focuses on processing sensory information such as touch and spatial awareness.
• The temporal lobe is associated with hearing and language.
• The occipital lobe processes visual information.

Figure 3.11 These are the four lobes in each hemisphere of the cerebral cortex. (CC BY 4.0; Rice University & OpenStax)

The lobes of the brain are further specialized. For example, the prefrontal cortex, in the front of the frontal lobe, helps regulate and control emotions as well as planning and judgment, and it continues to develop into early adulthood. The visual cortex, in the back of the occipital lobe, receives sensory input from the eyes and helps us perceive and integrate visual information. For example, this is where you might process color. The frontal lobe and parietal lobe also share responsibility for coordinating muscle movement and sensations such as touch, temperature, and pain via the motor and somatosensory cortices.

Sensitive periods are times in brain development during which certain areas develop more quickly. During sensitive periods, the brain is particularly susceptible to environmental influences. For example, the visual cortex undergoes a growth spurt at age two to four months, and most of the visual areas of the brain are developed by twelve months. If an infant is born with cataracts, their ability to see detail is much better if the cataracts are removed shortly after birth (before the sensitive period) rather than later (Maurer et al., 1999).

The process in which different functions become localized to one of the brain’s two hemispheres is called lateralization. Typically, the right side of the brain controls the left side of the body, and the other way around. So, our right visual field of view is processed in the left hemisphere and vice versa. Lateralization is influenced by both prenatal and after-birth experiences and governs handedness, language skill, and other traits. With maturation, we typically see language production and perception governed predominantly by the left hemisphere, with the right hemisphere’s role decreasing during childhood (Olulade et al., 2020).

Plasticity

The brain’s ability to change, physically and chemically, in response to environmental input and to compensate for injury is known as plasticity. This remarkable ability comes from the growth of new neurons, as well as the growth and pruning of synapses. Because of the rapid synaptogenesis happening during infancy, the brain is remarkably flexible in response to environmental inputs. This plasticity explains why younger children can often learn the sounds of a new language more quickly than older children or adults. However, plasticity may also make the brain more vulnerable to negative events (DeMaster et al., 2019). For example, infants born preterm are more vulnerable to inflammation and reduced oxygen supplies. But at the same time, plasticity allows the brain to recover more quickly from damage. For example, children who have had brain surgery to resolve severe epileptic seizures need less health-care intervention later (Pan et al., 2021).

When we consider all these developments together, we recognize that important changes in the brain are the result of environmental experiences. However, at the same time, new capabilities due to brain development allow infants and toddlers to explore and experience their environment, creating a feedback loop between the brain and the environment.

Health Risks in Infancy and Toddlerhood

The occurrence of death before age one year, known as infant mortality, can be a marker for the overall health of a society (CDC, 2024). In 2020, the U.S. infant mortality rate was 5.4 deaths per 1,000 births, compared to 1.5 in Slovenia, the nation with the world’s lowest rate (Central Intelligence Agency [CIA], 2023). However, even within the United States, racial and socioeconomic disparities in infant mortality occur. For example, the rate ranges from as low as 2.3 per 1,000 live births for Chinese American infants to 8.5 for American Indian/Alaska Natives and 11.2 for Black infants (Singh & Yu, 2019). Infant mortality is also higher for those living in high-poverty areas (Singh et al., 2017). These disparities are likely due to risks associated with economic and community inequalities such as reduced access to high-quality health care (Singh & Yu, 2019).

Globally, the leading causes of death in infants, toddlers, and young children are premature birth, respiratory infection, birth trauma, and malaria (Villavicencio et al., 2024). In lower-income countries with higher infant mortality rates, health risks for children aged two years and under include malaria, diarrheal diseases, and pneumonia (Ali & Aziz, 2021; WHO, 2022). Many causes of early childhood death can be prevented through vaccinations, adequate nutrition, and clean drinking water.

Vaccine-Preventable Diseases

Infants and toddlers are at risk for several vaccine-preventable diseases during the first two years of life (Table 3.4). The Centers for Disease Control and Prevention (CDC) recommend the vaccines U.S. children should receive, when they should receive them, and the time between doses (if applicable). All these vaccines have been tested and are provided for the safety and healthy development of the infant. Following the recommended schedule protects against the risk of serious illness and complications.

Table 3.4 Vaccine-Preventable Diseases (source: CDC, 2023b)

Vaccine	Preventable Disease(s)
Hepatitis B	Infants can contract hepatitis B from infected mothers during the birth process or through exposure to infected bodily fluids. The hepatitis B vaccine is recommended for all infants and is typically given within the first twenty-four hours after birth.
Rotavirus	Rotavirus is a common cause of diarrhea and vomiting in infants and young children. The rotavirus vaccine is recommended for all infants and is typically given in a series of two or three doses, depending on the vaccine used.
Diphtheria, tetanus, and pertussis (DTaP)	These bacterial infections can cause serious illness in infants and young children. The DTaP vaccine, which protects against all three, is recommended for all infants and is typically given in a series of five doses.
Polio	Polio is a viral infection that can cause paralysis and death. The polio vaccine is recommended for all infants and is typically given in a series of four doses.
Haemophilus influenzae type B (Hib)	This bacterial infection can cause serious illnesses such as meningitis and pneumonia in infants and young children. The Hib vaccine is recommended for all infants and is typically given in a series of three or four doses.
Pneumococcal disease	This bacterial infection can cause a range of illnesses, including pneumonia, meningitis, and ear infections. The pneumococcal vaccine is recommended for all infants and is typically given in a series of four doses.
Measles-mumps-rubella (MMR)	These are three infections that can cause serious illness or even death in infants and young children. The MMR vaccine protects against all three illnesses and is given in two doses (at twelve to fifteen months and again at four to six years of age).
Varicella	The varicella vaccine prevents chicken pox and is administered in two doses (at twelve to fifteen months and again at four to six years of age).

Malnutrition

Malnutrition, a lack of healthy nutrition, includes undernutrition, a condition in which calories and nutrients are missing, is responsible for 45 percent of all child deaths worldwide (WHO, 2022). Even without mortality, undernutrition is a serious health risk for infants and young children. Severe malnutrition reduces the number of neurons and synapses, dendrite growth, and myelination (Georgieff, 2007; Prado & Dewey, 2014).

Children in developing and war-torn countries are at risk for undernutrition. Starvation due to lack of calories and protein is marasmus, while kwashiorkor typically occurs after weaning in children whose diets have insufficient protein. Children then experience a loss of appetite and swelling of the abdomen as the body breaks down organs as a source of protein. Untreated, both marasmus and kwashiorkor can irreversibly delay physical and neurological development, leading to stunted growth, impaired cognition, and death (Alou et al., 2021; Bunker & Pandey, 2021).

Diarrheal disease, typically the result of an infection in the intestinal tract spread through contaminated food or water or poor hygiene, can also lead to malnutrition. It is the third leading cause of death in young children (WHO, 2024) because it can quickly lead to severe dehydration when access to health care is limited. Public health prevention and intervention programs implemented during pregnancy and for children under age two years can reduce risks for malnutrition and diarrheal disease (Rueda-Guevara et al., 2021).

Finally, malnutrition can involve overnutrition that is the result of eating too many nutrients. This can lead to overweight or obesity, an increasing problem in middle- and high-income countries that was exacerbated by increased food insecurity during the COVID-19 pandemic (Zemrani et al., 2021). It is associated with both short- and long-term health consequences, including cardiovascular disease, diabetes, and metabolic and endocrine disorders (Calcaterra & Zuccotti, 2022).

Link to Learning

Proper nutrition is critical for optimal development during childhood, but often children in poverty have limited access to high-quality nutrition. The UNICEF website provides information about the impact of poverty on malnutrition among children across the world.

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Footnotes

• 1This study (Barry, 2021) uses the term "Western."
• 2This study (Giannotti & Cortesi, 2009) uses the term “Anglo European.”
• 3This study (Bains & Mittal, 2023) uses the term "Western."