What Have You Learned?

1. Question 5.1

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   Weight gain in the first two years is dramatic. In the first few days of life, newborns typically lose a few ounces and then they gain an ounce a day for several months. Birthweight typically doubles by 4 months and triples by a year. Physical growth in the second year is slower but still rapid. By 24 months, most children weigh almost 28 pounds (13 kilograms). They have added more than a foot in height—from about 20 inches at birth to about 34 inches at age 2.

2. Question 5.2

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   The cells of the central nervous system are called neurons. Each neuron has a single axon and numerous dendrites, which spread out like the branches of a tree, making connections with the dendrites and axons of other neurons. Neurons communicate by sending electrochemical impulses through their axons to synapses (neuronal intersections) to be picked up by the dendrites of other neurons. The dendrites bring messages to the cell bodies of their neurons, which, in turn, convey the messages via their axons to the dendrites of other neurons.

3. Question 5.3

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   The expansive growth of dendrites is followed by pruning, in which unused brain connections atrophy and die. This loss of dendrites is important for the initial organization of the brain, and it increases brainpower.

4. Question 5.4

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   Infants cry as a reflex to pain, but they are too immature to decide to stop crying.

5. Question 5.5

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   Hours of sleep decrease rapidly with maturity: The norm per day for the first two months is 14¼ hours; for the next three months, 13¼ hours; for six to 17 months, 12¾ hours.

6. Question 5.6

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   Sensation is when a sensory system detects a stimulus. It precedes perception, which is the processing of a sensation. Perception leads to cognition, which is thinking about what has been perceived.

7. Question 5.7

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   Newborns are legally blind. Almost immediately, experience combines with maturation of the visual cortex to improve the ability to see shapes and then notice details. By 2 months, infants not only stare at faces but also, after perception and then cognition, smile. As perception builds, visual scanning improves. Thus, 3-month-old babies look closely at the eyes and mouth of a person, smiling more at smiling faces than at angry or expressionless ones. Binocular vision also develops between 2 and 4 months.

8. Question 5.8

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   During the first year, infants develop the ability to sit, crawl, walk (if not independently, then around the furniture, holding on), and increasingly control their heads, upper bodies, arms, and finally their legs and feet.

9. Question 5.9

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   Newborns have a strong reflexive grasp but lack control. By 3 months, infants can touch objects dangling within reach, but they cannot yet grab and hold on unless an object is placed in their hands. By 4 months, infants sometimes grab, but their timing is off; they close their hands too early or too late. Finally, by 6 months most babies can reach, grab, and grasp almost any object that is of the right size. Toward the end of the first year and throughout the second, finger skills improve as babies master the pincer movement and self-feeding. In the second year, grasping becomes more selective. Toddlers learn when not to grab things.

10. Question 5.10

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    Public health measures such as clean water, nourishing food, and immunizations are the main reasons for a decrease in infant mortality rates. Other measures that could lead to a further decrease include the availability of doctors and nurses in underserved areas.

11. Question 5.11

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    Immunization stimulates the body’s immune system to defend against an attack by a particular contagious disease and can be accomplished either naturally (by having the disease) or through vaccination.

12. Question 5.12

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    Each vaccinated child stops transmission of the disease and thus protects others, a phenomenon called herd immunity. Usually if 90 percent of the people in a community (a herd) are immunized, the disease does not spread to those who are vulnerable.

13. Question 5.13

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    Parents’ fears that immunizations can cause autism have led to reluctance to immunize infants. Any negative effects of an immunization get reported in the media, fueling parents’ concerns. However, several studies have refuted the link between immunization and autism, and the risk of a negative side effect of an immunization is minute compared to the risks associated with contracting the disease.

14. Question 5.14

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    Breast-feeding has many benefits, including the fact that babies who are exclusively breast-fed are less often sick. In infancy, breast milk provides antibodies against any disease to which the mother is immune and decreases allergies and asthma. Babies who are exclusively breast-fed for six months are less likely to become obese and thus less likely to develop diabetes or heart disease. Formula feeding is preferable only in unusual cases, such as when the mother is HIV-positive or uses toxic or addictive drugs.

15. Question 5.15

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    Chronically malnourished infants and children suffer in three ways: a) their brains may not develop normally, b) malnourished children have no body reserves to protect them against common diseases, and c) some diseases result directly from malnutrition, including marasmus during the first year and kwashiorkor after age 1.