The average weight of a normal newborn at birth is 3 kg. 3 months after birth, the infant’s weight is about twice that at birth. 12 months of age, when the infant’s weight is about three times that at birth, is the period of fastest weight gain after birth. To facilitate clinical application, there are formulas for rough weight estimation: weight of infants ≤ 6 months = 0.7 × age at birth + weight at birth. weight of infants 7-12 months = 0.25 × age at birth + 6. weight of infants 1 year to prepubertal age = 2 × age + 8. length of newborns at birth averages about 50 cm, with the fastest growth of about 25 cm in the first year and a slightly slower growth of about 10 cm in the second year. The formula for estimating height from 2 to 12 years old is: height = 7 × age + 75.