4.2: Physical Development During Infancy

We begin our exploration of postnatal development with an in-depth look at infancy and toddlerhood. We'll consider the many dramatic changes that take place from birth until about two years of age. Researchers have given this part of the life span more attention than any other period, perhaps because changes during this time are so dramatic and so noticeable - and perhaps because we have assumed that what happens during these years provides a foundation for one’s life to come. However, it has been argued that the significance of development during these years has been overstated. Nevertheless, this is a period of life that contemporary educators, healthcare providers, and parents have focused on most heavily. We will examine growth and nutrition during infancy, cognitive development during the first 2 years, the abilities and preferences of the neonate, and then turn our attention toward attachments formed in infancy.

Overall Physical Growth

The average newborn in the United States weighs about 7.5 pounds and is about 20 inches in length. For the first few days of life, infants typically lose about 5 percent of their body weight as they eliminate waste and get used to feeding. This often goes unnoticed by most parents/caregivers, but can be cause for concern for those who have a smaller infant. This weight loss is usually temporary, however, and is followed by a rapid period of growth. By the time an infant is 4 months old, they usually double in weight and by one year have tripled the birth weight. By age 2, the weight has quadrupled. The average length of the child at one year is about 26-32 inches.

Figure \(\PageIndex{1}\): At what age can a baby wink at you - with meaning? (Unsplash license; Omar Lopez via Unsplash)

Body Proportions

Another dramatic physical change that takes place in the first several years of life is the change in body proportions. The head initially makes up about 50 percent of our entire length when we are developing in the womb. At birth, the head makes up about 25 percent of our length (think about how much of your length would be head if the proportions were still the same!). By age 25 it comprises about 20 percent our length. Imagine now how difficult it must be to raise one’s head during the first year of life! And indeed, if you have ever seen a 2 to 4 month old infant lying on the stomach trying to raise the head, you know how much of a challenge this is.

The Brain in the First Two Years

Some of the most dramatic physical change that occurs during this period is in the brain. At birth, the brain is about 25 percent of its adult weight and this is not true for any other part of the body. By age 2, it is at 75 percent its adult weight, at 95 percent by age 6 and at 100 percent by age 7 years.

While most of the brain’s 100 to 200 billion neurons are present at birth, they are not fully mature and during the next several years dendrites or connections between neurons will undergo a period of transient exuberance or temporary dramatic growth. Neurons, or nerve cells, are the smallest functional unit in the nervous system. The simplest of neurons consist of a cell body that has extensions called dendrites that receive information from other neurons. And an axon that carries information away from the cell body. The exchange of information between neurons happens at synapses.

Figure \(\PageIndex{2}\): In a neuron, dendrites receive information and axons send information. (CC BY-SA; NA via US National Cancer Institute's SEER Program)

There is a proliferation of dendrites during the first two years so that by age 2, a single neuron might have thousands of dendrites. After this dramatic increase, the neural pathways that are not used will be eliminated (a process called synaptic pruning which refers to the process by which extra neurons and synaptic connections are eliminated in order to increase the efficiency of neuronal transmissions) thereby making those that are used much stronger. This activity is occurring primarily in the cortex (the outer covering of the brain). The prefrontal cortex that is located behind our forehead continues to grow and mature throughout childhood and experiences an additional growth spurt during adolescence. It is the last part of the brain to mature and will eventually comprise 85 percent of the brain’s weight. Experience will shape which of these connections are maintained and which of these are lost. Ultimately, about 40 percent of these connections will be lost (Webb, Monk, and Nelson, 2001). As the prefrontal cortex matures, the child is increasingly able to regulate or control emotions, to plan activity, strategize, and have better judgment. Of course, this is not fully accomplished in infancy and toddlerhood (not by a long stretch!), but continues throughout childhood and adolescence.

Another major change occurring in the central nervous system is the development of myelin, a coating of fatty tissues around the axon of the neuron. Myelin helps insulate the nerve cell and speed the rate of transmission of impulses from one cell to another. This enhances the building of neural pathways and improves coordination and control of movement and thought processes. The development of myelin continues into adolescence but is most dramatic during the first several years of life.

From Reflexes to Voluntary Movements

Infants are equipped with a number of reflexes which are involuntary movements in response to stimulation. These include the sucking reflex (infants suck on any objects that touch their lips automatically), the rooting reflex, which involves turning toward any object that touches the cheek, the palmar grasp (the infant will tightly grasp any object placed in its palm), and the stepping reflex (also called the dancing reflex) that happens when the infant is held up in a standing position and moves their feet up and down alternately as if stepping or dancing. These movements occur automatically and are signals that the infant is functioning well neurologically. Within the first several weeks of life these reflexes are replaced with voluntary movements or motor skills.

Primitive Reflexes (wikipremed.com)

Primitive reflexes are reflex actions originating in the central nervous system that are exhibited by normal infants, but not neurologically intact adults, in response to particular stimuli.

• A reflex action by which an infant turns its head toward anything that strokes its cheek or mouth, the rooting reflex is present at birth and disappears around four months of age.
• The Moro reflex is an infantile reflex normally present in all infants/newborns up to 4 or 5 months of age as a response to a sudden loss of support. It involves three distinct components: spreading out the arms, unspreading the arms, and crying.
• An upward response (extension) of the big toe when the sole of the foot is stimulated with a blunt instrument is known as the Babinski sign (or reflex), which can identify disease of the spinal cord and brain in adults, and also exists as a primitive reflex in infants.

Babinski's Sign in a healthy newborn.

Lifelong Reflexes (medlineplus.gov)

The presence and strength of a reflex is an important sign of nervous system development and function. Many infant reflexes disappear as the child grows older, although some remain through adulthood. A reflex that is still present after the age when it would normally disappear can be a sign of brain or nervous system damage.

Examples of reflexes that last into adulthood are:

• Blinking reflex: blinking the eyes when they are touched or when a sudden bright light appears
• Cough reflex: coughing when the airway is stimulated
• Gag reflex: gagging when the throat or back of the mouth is stimulated
• Sneeze reflex: sneezing when the nasal passages are irritated
• Yawn reflex: yawning when the body needs more oxygen

Gross Motor Skills

These voluntary movements involve the use of large muscle groups and are typically large movements of the arms, legs, head, and torso. These skills begin to develop first. Examples include moving to bring the chin up when lying on the stomach, moving the chest up, rocking back and forth on hands and knees. But it also includes exploring an object with one’s feet as many babies do as early as 8 weeks of age if seated in a carrier or other devise that frees the hips. This may be easier than reaching for an object with the hands, which requires much more practice. And 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!

Fine Motor Skills

Fine motor skills are more exact movements of the hands and fingers and include the ability to reach and grasp an object. Newborns cannot grasp objects voluntarily but do wave their arms toward objects of interest. At about 4 months of age, the infant is able to reach for an object, first with both arms and within a few weeks, with only one arm. Grasping an object involves the use of the fingers and palm, but no thumbs. Stop reading for a moment and try to grasp an object using the fingers and the palm. How does that feel? How much control do you have over the object? If it is a pen or pencil, are you able to write with it? Can you draw a picture? The answer is probably not. Use of the thumb comes at about 9 months of age when the infant is able to grasp an object using the forefinger and thumb. This ability greatly enhances the ability to control and manipulate an object and infants 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. This is quite a complicated set of actions if we remember how difficult this would have been just a few months earlier.