The birth weight of newborns is related to their gestational age, sex and intrauterine nutritional status. The average birth weight of male infants was 3.33 kg ± 0.39 kg and that of female infants was 3.24 kg ± 0.39 kg. Subsequent weight gain gradually decreases with increasing age. The first year of life is the first peak of postnatal weight gain and is the fastest period of weight gain in a lifetime.
Height represents the length of the head, spine and lower limbs. Most infants under 3 years of age are measured for length, and because it is not easy to measure accurately in the standing position, they are measured in the supine position. The normal difference between the standing and supine measurements is 0.7 cm due to different measurement positions and methods. 2 to 3 years old can measure both length and height, corrected by 0.7. The first year after birth will also face the first peak in length growth, and is also the fastest period of height growth in a lifetime. The average length at birth is about 50 cm; the length growth in the first year after birth is about 25 cm, and the length at the age of 1 year is about 1.5 times the length at birth. The growth pattern of length in the first year is basically parallel to that of weight, and the growth of length in the first 3 months is basically the same as that in the last 9 months.
The growth of head circumference is related to the development of brain and skull. The head circumference at birth is about 32-34 cm, slightly larger than the chest circumference by 1-2 cm. The growth of head circumference in the first 3 months of the first year is about the same as the growth of head circumference in the last 9 months. The head circumference at 1 year old is about 46 cm. The chest circumference represents the development of lung and thorax. At birth, chest circumference is slightly smaller than head circumference, and at 1 year of age, chest circumference is approximately equal to head circumference.
In addition, there are also measurements of upper arm circumference, sitting height and finger spacing. These indicators not only reflect the rate of physical growth, but are also important in assessing whether the body is proportionate. At each age, the body should show the corresponding proportion, such as infant head about 1/4 of the height, gradually decreasing with age, the proportion of adults about 1/8; sitting height at birth accounted for about 0.67 of the height (long), gradually decreasing with age, the proportion of 6 years old is about 0.56; normal people finger distance should be slightly smaller than the height (long). The imbalance of the above ratio may be a reflection of certain diseases, and the cause should be promptly investigated.