Infancy period, traditionally, is the time from 1 month to 1 year of age. Because of the growth and learning potential that occurs, the first year of life is a crucial one.
Physical development during the infancy period
- Weight. Most infants double their birth weight at 4 to 6 months and it triples at 1 year. A weight gain of 2 lb per month during the first 6 months is typical among infants. During the second 6 months, the expected weight gain is approximately 1 lb per month.
- Height. Height like weight is ideally assessed if plotted on a standard growth chart. The average birth length of an infant is about 20 inches. During the first year an infant increases in height by about 50 %. In early months the growth is apparent in the trunk while it becomes more apparent as lengthening of the legs during the second half of the first year.
- Head circumference. Rapid brain growth is reflected by a rapidly increasing head circumference. By the end of the first year of life, the brain has already reached two-thirds of its adult size.
- Teeth. By 6 months of age, the first baby tooth which is typically ca central incisor erupts. The eruption is followed by a new one monthly. Usual ages of baby tooth eruption by tooth type are as follows:
- Central lower incisor – 6 to 10 months
- Upper central incisor – 8 to 12 months
- Upper lateral incisor – 9 to 13 months
- Lower lateral incisor – 10 to 16 months
- Upper cuspid – 16 to 22 months
- Lower cuspid – 17 to 23 months
- Upper first molar – 13 to 19 months
- Lower first molar – 14 to 18 months
- Lower second molar – 23 to 31 months
- Upper second molar – 25 to 33 months
- Gross motor development. To assess the development of gross motor skills in infants, four positions should be observed. The positions are as follows:
Ventral suspension position
This is the appearance of the infant when held in midair on a horizontal plane, supported by a hand under the abdomen. With this position, the infant’s head is allowed to hang down with little effort on control.
- One month old infants lift the head momentarily and drop it again. Flexion of elbows, hip extension and knee flexion may be observed.
- By two months of age, the head is held in the same place as the rest of their body. This signifies a major advancement in muscle control.
- A month after, the head is lifted and maintained well above the plane of the rest of the body in ventral suspension. It is at this month where a Landau reflex is developed. Landau reflex is the extension of the infant’s head, legs and spine when held in ventral suspension. Most infants continue to present this reflex until 6 months of age. Inability to perform this reflex would suggest further evaluation for possible motor weakness, cerebral palsy, or other neuromuscular defect.
- When Landau reflex diminished, the infant then demonstrates a parachute reflex. By 6 to 9 months of age, infants suddenly lower towards the examining table while extending the arms as if protecting themselves when held in a ventral suspension position. An inability to demonstrate this reflex would suggest cerebral palsy because they flex their extremities too tight.