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The Merck Manual--Second Home Edition logo
 
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Chapter 263. Normal Newborns and Infants
Topics: Introduction | Initial Care | Physical Examination | First Few Days | Feeding | Stools and Urine | Sleeping | Physical Development | Behavioral, Social, and Intellectual Development | Promoting Optimal Development | Preventive Health Care Visits | Vaccinations
 
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Physical Development

An infant's physical development depends on heredity, nutrition, and environment. Physical and psychologic abnormalities can also influence growth. Optimal growth requires optimal nutrition and health.

A newborn normally loses 5 to 7% of his birth weight during the first few days of life. Newborns who are breastfeeding can lose up to 7% of their birth weight. This weight is regained by the end of the first 2 weeks as the newborn starts to eat more. After this, an infant typically gains around one ounce per day during the first two months, and a pound per month after that. This generally results in a doubling of birth weight by age 5 months and a tripling by 1 year. An infant's length increases about 30% by age 5 months and more than 50% by 1 year.

click here to view the figure See the figure An Infant's First Year: Physical Development.

Different organs grow at different rates. For example, the reproductive system has a brief growth spurt just after birth, then changes very little until just before puberty. In contrast, the brain grows almost exclusively during the early years of life. At birth, the brain is one fourth of its future adult size. By 1 year, the brain is three fourths of its adult size. The kidneys function at the adult level by the end of the first year.

Lower front teeth begin to appear at the age of 5 to 9 months. Upper front teeth begin to appear at 8 to 12 months.

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