Summary

1. In the first two years of life, infants grow taller, gain weight, and increase in head circumference—all indicative of development. Birthweight doubles by 4 months, triples by 1 year, and quadruples by 2 years, when toddlers weigh about 28 pounds (12.7 kilograms).
2. Brain size increases even more dramatically, from about 25 to 75 percent of adult weight in the first two years. Complexity increases as well, with cell growth, development of dendrites, and formation of synapses. Both growth and pruning aid cognition. Experience is vital for brain development.
3. The amount of time a child sleeps gradually decreases over the first two years. As with all areas of development, variations in sleep patterns are normal, caused by both nature and nurture. Bed-sharing is the norm in many developing nations, and co-sleeping is increasingly common in developed ones.

4. At birth, the senses already respond to stimuli. Prenatal experience makes hearing the most mature sense. Vision is the least mature sense at birth, but it improves quickly. Infants use all their senses to strengthen their early social interactions.
5. Infants gradually improve their motor skills as they begin to grow and brain maturation increases. Gross motor skills are soon evident, from rolling over to sitting up (at about 6 months), from standing to walking (at about 1 year), from climbing to running (before age 2).
6. Babies gradually develop the fine motor skills to grab, aim, and manipulate almost anything within reach. Experience, time, and motivation allow infants to advance in all their motor skills.

7. About 2 billion infant deaths have been prevented in the past half-century because of improved health care. One major innovation is immunization, which has eradicated smallpox and virtually eliminated polio and measles. More medical professionals are needed to prevent, diagnose, and treat the diseases that still cause many infant deaths in poor nations.
8. Breast-feeding is best for infants, partly because breast milk helps them resist disease and promotes growth of every kind. Most babies are breast-fed at birth, but in North America only one-third are exclusively breast-fed for three months, as doctors worldwide recommend.
9. Severe malnutrition stunts growth and can cause death, both directly through marasmus or kwashiorkor and indirectly through vulnerability if a child catches measles, an intestinal virus, or some other illness.
10. Careful scientific research and multicultural awareness have led to a dramatic reduction worldwide in sudden infant deaths (SIDS). The specific practice that has saved thousands of infants is putting babies to sleep on their backs, not their stomachs.