Now that we have looked at the ways in which infants and toddlers develop cognitively and linguistically, let’s take a look at how the body changes during the first 3 years of life. Physical development includes brain growth, skeletal growth, muscle development, coordination development, and development of gross and fine motor skills. There are many physical changes that take place during this time. For example, newborn babies, who are relatively immobile, will begin to roll over in just a few months and eventually start moving about the world.

We can witness the physical changes of the body by observing how the child fits into their clothing or the way the child is able to move independently. Other changes occur within the complex structures of the brain. While we cannot see this transformation happen, we certainly notice it when babies start to engage in increasingly advanced tasks, such as crawling and walking. Let’s take a look at what’s happening in a child’s early physical development by examining the growing body.

The Growing Body

The body goes through dramatic physical changes in the first several years of life, starting first with the brain and head. A young child’s head is relatively big compared to the rest of their body. The head initially makes up about 50 percent of the body’s length when we are developing in the womb. At birth, the brain is 25 percent of its adult size, 75 percent by age 1, and 80 percent by age 3 (Graber, 2023).

Infants have a difficult time holding their head up due to immature neck muscles and their large heads. As they start to explore their bodies and attempt to move around, the neck muscles eventually become strong enough to support other movements. Infants can usually hold their heads up at around two months old, especially if they are given tummy time, which is the practice of placing an infant on a flat surface on their stomachs during awake times. The World Health Organization recommends that infants get at least 30 minutes of tummy time daily (WHO, 2022).

Newborn reflexes will switch from automatic responses to those of intention. Infants develop strong sensory responses, which then prompt physical responses to stimuli within the environment. For example, a caregiver may place a baby on a play mat with several small objects hanging over it. At first, the infant may only be able to observe the objects. Eventually, they will be able to grab or swat at them. These seemingly simple actions are important in the overall development of a child’s physical skills.

Infants experience quick body growth in the first year of life. For example, they will see an increase in body length of 30 percent by 5 months old and more than 50 percent by a year old (Graber, 2023). Children’s bodies will continue to be characterized by big heads, curved backs, and a protruding belly. They have short arms and legs and may appear bow-legged (Lee, 2023; Graber, 2023). By 3 years of age, children will have more proportionate bodies as they gain more muscle and grow taller.

A big change that is occurring within a child’s body is the ability to control the muscles associated with potty training. “Most children develop control over their bowel and bladder by 18 months” (Zero to Three, 2010). A child’s physical capabilities are important in potty training, but a child may not be emotionally ready to handle potty training. Toddlers are becoming increasingly independent and demonstrate the need for control over their bodies. Making potty training a natural part of their learning process will help caregivers avoid the infamous toddler power struggles. Children may decide to exert control over their bodies by not going to the potty when needed, which can lead to physical issues such as constipation (Zero to Three, 2010).

There are many factors that impact a child’s physical development. Genetic and heritable traits are passed from parents to children and can influence the way a body is constructed and developed. Environmental factors, such as toxins or teratogens, can also impact whether a child’s brain and body develop as expected. A child’s access to healthy and consistent nutrition influences whether a child acquires the vitamins and nutrients needed to support their brain and body growth. Most importantly, the types of interactions that infants and toddlers have with their primary caregivers serve as a foundation for healthy physical development.

Motor Development

Motor development occurs in an orderly sequence as infants move from reflexive reactions (e.g., sucking and rooting) to more advanced motor functioning in toddlerhood. As mentioned during the prenatal section, development occurs according to the cephalocaudal (from head to tail) and proximodistal (from the midline outward) principles. For instance, babies first learn to hold their heads up, then to sit with assistance, then to sit unassisted, followed later by crawling, pulling up, cruising or walking while holding on to something, and then unassisted walking (Eisenberg, Murkoff, & Hathaway, 1989).

As motor skills develop, there are certain developmental milestones that young children should achieve. An example of a developmental milestone is a baby holding up its head. Babies on average are able to hold up their heads at 6 weeks old, and 90 percent of babies achieve this between 3 weeks and 4 months old. On average, most babies can sit alone at 7 months old. Sitting involves both coordination and muscle strength, and 90 percent of babies achieve this milestone between 5 and 9 months old. If the child is displaying delays in reaching multiple milestones, that is reason for concern, and the parent or caregiver should discuss this with the child’s pediatrician. Developmental delays can be identified and addressed through early intervention.

Motor skills refer to our ability to move our bodies and manipulate objects. Fine motor skills focus on the muscles in our fingers, toes, and eyes, and enable coordination of small actions (e.g., grasping a toy, writing with a pencil, and using a spoon). Newborns cannot grasp objects voluntarily, but they do wave their arms toward objects of interest. At about 4 months of age, the infant is able to reach for an object. They first reach with both arms, but within a few weeks they are reaching with only one arm. At this age, grasping an object involves the use of the fingers and palm, but no thumbs. This is known as the palmer grasp.

The use of the thumb begins at about 9 months of age. Now the infant uses a pincer grasp, grasping an object using the forefinger and thumb. This ability greatly enhances infants’ ability to control and manipulate an object, and they take great delight in this newfound ability. They may spend hours picking up small objects from the floor and placing them in containers. By 9 months, an infant can also watch a moving object, reach for it as it approaches, and grab it.

Gross motor skills utilize the large muscle groups that control our head, torso, arms, and legs and involve larger movements (e.g., balancing, running, and jumping). These skills begin to develop first. Examples include moving to bring the chin up when lying on the stomach, moving the chest up, and rocking back and forth on hands and knees. But it also includes exploring an object with one’s feet, which many babies do as early as 8 weeks of age if they are seated in a carrier or other device that frees the hips. This may be easier than reaching for an object with the hands, which requires much more practice (Berk, 2007). Sometimes an infant will try to move toward an object while crawling and surprisingly move backward because of the greater amount of strength in the arms than in the legs.

Licenses and Attributions for Physical Development

“Physical Development” by Christina Belli is licensed under CC BY 4.0.

“Motor Development” from Lifespan Development – A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under CC BY 4.0; minor edits.