16 Unit 5: Physical Development – Foundations Of Child And Adolescent Psychology

Physical Development during Infancy

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, but can be cause for concern for those who have a smaller infant. This weight loss is temporary, however, and is followed by a rapid period of growth. By the time an infant is 4 months old, it usually doubles in weight and by one year has tripled it birth weight. By age 2, the weight has quadrupled. The average length at one year is about 26-32 inches.

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. By age 25 it comprises about 20 percent of 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. (39)

Sudden Unexpected Infant Death and Sudden Infant Death Syndrome

Sudden unexpected infant death (SUID) is a term used to describe the sudden and unexpected death of a baby less than 1 year old in which the cause was not obvious before investigation. These deaths often happen during sleep or in the baby’s sleep area.

About 3,500 babies in the United States die suddenly and unexpectedly each year. A thorough investigation is necessary to learn what caused these deaths. Sudden unexpected infant deaths include sudden infant death syndrome (SIDS), accidental suffocation in a sleeping environment, and other deaths from unknown causes. Although the SUID rate has declined since 1990s, significant racial and ethnic differences continue. (40)

The breakdown of sudden unexpected infant deaths by cause in 2015 is as follows: 43% of cases were categorized as sudden infant death syndrome, followed by unknown cause (32%), and accidental suffocation and strangulation in bed (25%).
Figure 5-1: Breakdown of Sudden Unexpected Infant Death by Cause (2015) by the Centers for Disease Control and Prevention resides in the Public Domain .
This stacked bar chart shows sudden unexpected infant death (SUID) rates by cause (sudden infant death syndrome, unknown cause, and accidental suffocation and strangulation in bed) and by race/ethnicity in the U.S. from 2011 through 2014.
Figure 5-2: Sudden Unexpected Infant Death by Race/Ethnicity, 2011-2014 by the Centers for Disease Control and Prevention resides in the Public Domain .

Reducing the Risk of SIDS

Health care providers and researchers don’t know the exact causes of Sudden Infant Death Syndrome (SIDS). However, research shows parents and caregivers can take the following actions to help reduce the risk of SIDS and other sleep-related infant deaths:

  • Place the baby on his or her back for all sleep times — for naps and at night.
  • Use a firm sleep surface, such as a mattress in a safety-approved crib, covered by a fitted sheet.
  • Have the baby share the parents’ room, not their bed. The baby should not sleep in an adult bed, on a couch, or on a chair alone, or with anyone else.
  • Keep soft objects, such as pillows and loose bedding, out of the baby’s sleep area.
  • Parents should not smoke during pregnancy, or allow smoking around the baby. (41)

The Brain in the First Two Years

Some of the most dramatic physical changes that occur during this period is in the brain. At birth, the brain is about 25 percent 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. There is a proliferation of these 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 thereby making those that are used much stronger. This activity is occurring primarily in the cortex or the thin outer covering of the brain involved in voluntary activity and thinking. The prefrontal cortex that is located behind our forehead continues to grow and mature throughout childhood and experiences an addition 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, 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 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 dancing reflex evidence when the infant is held in a standing position and moves its feet up and down alternately as if 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.

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 (Berk, 2007). 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. (42)


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