Human Growth and Development
In the context of the physical development of children, growth refers to the increase in the size of a child, and development refers to the process by which the child develops his or her psychomotor skills.
The period of human growth from birth to adolescence is commonly divided into the following stages:
- Infancy: From birth to weaning.
- Childhood: From weaning to the end of brain growth.
- Juvenile: From the end of childhood to adolescence.
- Adolescence: From the start of growth spurt at puberty until sexual maturity.
Growth curves are used to measure growth. The distance curve is a measure of size over time; it records height as a function of age and gets higher with age. The velocity curve measures the rate of growth at a given time for a particular body feature (such as height or weight). The height velocity curve is highest in infancy, up to two years of age, with more consistent annual growth afterwards and increases again at puberty. The height of the average infant increases by 30% by the age of five months and by 50% by the age of one year. The height of a five-year-old usually doubles relative to that at birth. The limbs and arms grow faster than the trunk, so that body proportions undergo marked variation as an infant grows into an adolescent. Different body systems grow and develop at different rates. For example, if infants grew in height as quickly as they do in weight, the average one-year-old would be approximately 5 ft (1.5m) tall. Thus, weight increases faster than height—an average infant doubles his birth weight by the age of five months and triples it by the age of one year. At two years of age, the weight is usually four times the weight at birth.
During the growth period, all major body systems also mature. The major changes occur in the following systems:
- Skeletal system. At birth, there is very little bone mass in the infant body, the bones are softer (cartilagenous) and much more flexible than in the adult. The adult skeleton consists of 206 bones joined to ligaments and tendons. It provides support for the attached muscles and the soft tissues of the body. Babies are born with 270 soft bones that eventually fuse together by the age of 20 into the 206 hard, adult bones.
- Lymphatic system. The lymphatic system has several functions. It acts as the body's defense mechanism by producing white blood cells and specialized cells (antibodies) that destroy foreign organisms that cause disease. It grows at a constant and rapid rate throughout childhood, reaching maturity just before puberty. The amount of lymphatic tissue then decreases so that an adult has approximately 50% less than a child.
- Central nervous system (CNS). The CNS consists of the brain, the cranial nerves, and the spinal cord. It develops mostly during the first years of life. Although brain cell formation is almost complete before birth, brain maturation continues after birth. The brain of the newborn is not yet fully developed. It contains about 100 billion brain cells that have yet to be connected into functioning networks. But brain development up to age one is more rapid and extensive than was previously realized. At birth, the brain of the infant is 25% of the adult size. At the age of one year, the brain has grown to 75% of its adult size and to 80% by age three, reaching 90% by age seven. The influence of the early environment on brain development is crucial. Infants exposed to good nutrition, toys, and playmates have better brain function at age 12 than those raised in a less stimulating environment.
During the first year of life, a baby goes through a series of crucial stages to develop physical coordination. This development usually proceeds cephalocaudally, that is from head to toe. For example, the visual system reaches maturity earlier than do the legs. First, the infant develops control of the head, then of the trunk (sitting up), then of the body (standing), and, finally, of the legs (walking). Development also proceeds proximodistally, that is from the center of the body outward. For example, the head and trunk of the body develop before the arms and legs, and infants learn to control their neck muscles before they learn to direct their limbs. This development of physical coordination is also referred to as motor development and it occurs together with cognitive development, meaning the development of processes such as knowing, learning, thinking, and judging.
The stages of motor development in children are as follows:
- First year. The baby develops good head balance and can see objects directly in his line of vision. He learns how to reach for objects and how to transfer them from one hand to the other. Sitting occurs at six months of age. Between nine and 10 months, the infant is able to pull himself to standing and takes his first steps. By the age of eight to 24 months, the baby can perform a variety of tasks such as opening a small box, making marks with a pencil, and correctly inserting squares and circles in a formboard. He is able to seat himself in small chair, he can point at objects of interest, and can feed himself with a spoon.
- Second year. At 24-36 months, the child can turn the pages of a book, scribble with a pencil, build towers with blocks up to a height of about seven layers, and complete a formboard with pieces that are more complex than circles or squares. He can kick a ball, and walks and runs fairly well, with a good sense of balance. Toilet training can be started.
- Third year. The child can now draw circles, squares, and crosses. He can build 10-block towers and imitate the building of trains and bridges. He is also achieving toilet independence. Hand movements are well coordinated and he can stand on one foot.
- Four years. At that age, a child can stand heel to toe for a good 15 seconds with his eyes closed. He can perform the finger-to-nose test very well, also with eyes closed. He can jump in place on both feet.
- Five years. The child can balances on tiptoe for a 10-second period, he can hops on one foot, and can part his lips and clench his teeth.
- Six years. The child can balance on one foot for a 10-second period, he can hit a target with a ball from 5 ft (1.5 m), and jumps over a rope 8 in (20 cm) high.
- Seven years. He can now balance on tiptoes for a 10-second period, bend at the hips sideways, and walk a straight line, heel-to-toe for a distance of 6 ft (1.8 m).
- Eight years. The child can maintain a crouched position on tiptoes for a 10-second period, with arms extended and eyes closed. He is able to touch the fingertips of one hand with his thumb, starting with the little finger and repeating in reverse order.
The development of motor skills in the child goes hand in hand with the development of cognitive skills, a process called cognitive development. Cognitive development can be divided into four stages:
- Sensorimotor stage. At this stage, infants discover their environment using a combination of sensory impressions (sight, smell, hearing, taste, and touch) and motor activities.
- Preoperational stage. At this stage, children are not able to use information in rational and logical ways, rather they use images and symbols. They learn how to associate cause and effect and to represent something with something else. Speech development begins.
- Concrete operational stage. At this stage, children understand elementary logical principles that apply to concrete external objects. They learn to sort things into categories, reverse the direction of their thinking, and think about two concepts (such as length and width) simultaneously.
- Formal operational stage. This stage is reached at adolescence. The individual can think in the abstract and speculate about probabilities and possibilities as well as reflect on their own thinking activities.
The simultaneous development of motor skills and cognitive skills is commonly referred to as psychomotor development and it occurs with the maturation of the central nervous system (CNS).
Bilirubin—A pigment produced as the liver processes waste products. Fetal bilirubin is eliminated from the fetus by placental transfer into the mother's plasma. At birth, the infant's liver takes over the elimination of bilirubin.
Central nervous system (CNS)—In humans, the system that consists of the brain, the cranial nerves, and the spinal cord.
Cognitive skills—Skills required to perform higher cognitive processes, such as knowing, learning, thinking, and judging.
Endocrine system—The endocrine system is the collection of glands that produce hormones. Endocrine glands release hormones directly into the bloodstream, where they are transported to organs and tissues throughout the entire body.
Frontal lobes—The frontal lobes of the brain are responsible for higher cognitive processes, meaning the mental processes of knowing, learning, thinking, and judging.
Hormone—Specialized substances required for normal body functions and produced by the glands of the endocrine system. Hormones regulate metabolism, growth, and sexual development.
Human growth hormone (hGH)—Hormone produced by the pituitary gland in the brain. It is usually released during sleep in response to positive and negative signals from the hypothalamus. Also known as the master hormone of the body, hGH affects growth, development, immunity, and metabolism.
Hypothalamus—The hypothalamus is located in the brain, connected to the cerebral cortex, thalamus, and other parts of the brain stem so that it can receive impulses from them and send impulses to them. It thus functions as a link between the nervous and endocrine systems, being controlled by the central nervous system and controlling, in turn, the pituitary gland.
Immune system—The system that defends the body against infection, disease, and foreign substances.
Motor activity—The physical activity of an individual.
Motor cortex—The area of the frontal lobes of the brain concerned with primary motor control.
Motor skills—Skills required to perform complex motor acts, meaning acts that produce physical movement.
Nervous system—The nervous system is the entire system of nerve tissue in the body. It includes the brain, the brainstem, the spinal cord, the nerves, and the ganglia.
Placenta—An organ that joins the mother to the fetus and provides endocrine secretions as well as the capacity to exchange bloodborne substances, such as nutrients and waste products.
Psychomotor skills—Skills that develop with the maturation of the central nervous system and include both motor and cognitive skills.
Puberty—The period during which the secondary sexual characteristics begin to develop and at which the individual becomes capable of sexual reproduction.
Sense—A perception by the sensory organs of the body. The major senses are sight, smell, hearing, taste, and touch.
Sensory organs—Organs that allow the body to see, smell, hear, taste, and touch.
The function of postnatal growth and development is to bring the individual to the stage of healthy adulthood, physically characterized by the end of growth with full sexual maturity and fertility for the individual.
Role in human health
Successful growth and development promotes health, providing not only physical but also emotional and psychological well-being.
Common diseases and disorders
There are many possible reasons for the impairment of growth and development in a child. Growth and development depend on the interplay of several factors, such as the genetic make-up of the child, the completion of normal fetal development, the diet from time of birth, the normal development of the central nervous system, and the quality of the psychological and physical environment, to name but a few. Any disturbance in any of the factors required for growth and development will accordingly affect the successful outcome of the process.
Specific disorders affecting growth and development include:
- Neonatal disorders. During the nine months of gestation, life sustaining functions, such as supplying of oxygen and nutrients, the elimination of waste, and the regulation of body temperature, are all taken care of by the mother. At the moment of birth, the newborn must abruptly take over the performance of all these tasks. Neonatal disorders include all conditions resulting from the unsuccessful transition from fetus to newborn. They cover the wide range of all body systems that undergo significant change at birth. For example, the inability to change from placental to lung-based respiration may include respiratory distress syndrome (RDS), which can occur in premature infants, and several other breathing disorders. Problems with the blood circulation transition may, for example, result in inadequate oxygen intake (asphyxia) and/or in the decrease of oxygen supply to the tissues (hypoxia). Bilirubin excretion problems may also occur (jaundice) if the infant's liver can not adequately replace the mother's placenta. Immunological disorders or infections may also result due to the immaturity of the newborn's immune system.
- Feeding and gastrointestinal disorders. Infant feeding disorders include milk regurgitation, overfeeding, underfeeding, vomiting, diarrhea, constipation, colic, and adverse effects due to the presence of drugs, if any, in the mother's milk. There are also a number of gastrointestinal disorders that may lead to poor absorption and utilization of food by the body. Failure to absorb nutrients and energy from food then leads to growth deficiencies.
- Inadequate nutrition. Nutritional deficiencies will cause poor growth and development. A balanced diet with adequate calories and protein intake is essential for optimal growth. Children who do not eat proper foodstuffs (malnutrition) develop growth disorders accompanied by intellectual underachievement.
- Attention deficit disorder (ADD). ADD is characterized by the inability to concentrate, hyperactivity, irritability, and impulsivity. As of 2001, 3–10% of the nation's school-age children were diagnosed with the disorder.
- Human growth hormone (hGH) deficiencies. hGH, also known as the master hormone of the body, is responsible for regulating growth, development, immunity, and metabolism. It affects the growth of tissues, bones, cartilage, muscles, skin, liver, and kidneys. hGH deficiency results in increased body weight and abdominal obesity, decreased lean body mass and decreased muscle mass, decreased strength, poor sleep, decreased physical performance capacity, and lower cardiac performance.
- Diseases affecting the kidneys. Diseases of the kidneys may also impair growth and development as a result of buildup of waste products and undesirable substances in the body. For example, diabetic children grow slowly if their blood sugar is not maintained in the normal range.
- Disorders of the nervous system. Disorders affecting the nervous system can occur before or after birth with diagnosis usually made before one year of age. Some of the conditions that may affect growth and development include partial paralysis (spastic paresis), seizures (often infantile spasms), and abnormally large head (macrocephaly) or small head (microcephaly). Infants with nervous system disorders may have poor or absent speech development, epilepsy, abnormal fluid accumulation in the brain and skull (hydrocephalus), shrinkage or shortening of muscle tissue (spastic contractures), and mental retardation.
- Genetic or chromosomal abnormalities. There are many genetic disorders that may cause growth failure in children or affect it more or less seriously. They include hereditary defects incompatible with long-term development and survival, as well as hereditary conditions that are seriously life-threatening or that impair some aspect of growth and development.
- Severe stress or emotional deprivation. The child's psychological environment also affects physical development. A child requires care, affection, and stimulation for the normal growth and development of his body, brain, and nervous system. A striking example is provided by failure-to-thrive syndrome, in which children suffering from prolonged neglect or abuse simply stop growing. In these children, the psychological stress produced by their social environment causes the endocrine system to stop secreting growth hormones.
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Human Growth Foundation, 997 Glen Cove Ave., Glen Head, NY 11545. email@example.com.
National Institute of Child Health and Human Development, Building 31, Room B2B15, 9000 Rockville Pike, Bethesda, MD 20892.
The National Parenting Center. "Physical Aspects of Infancy." <http://www.tnpc.com/parentalk/infancy.html>.
Monique Laberge, PhD