A child grows faster during babyhood than at any other stage of its life, including adolescence. By the age of 18 months a girl is usually half her adult height, while a boy is half their adult height by the age of two years.
There is little correlation between the rate of growth in childhood and eventual height. Many children grow quickly and then stop early so that they are short, whereas others seem to grow at a slower pace but continue until they outstrip everyone else.
The most significant factor in determining height is heredity – the children of tall parents will usually also be tall. Nutrition is also significant, and a child who is poorly nourished is likely to be shorter than one who is well nourished. Advances in nutrition are the main reason for an overall increase in the height of populations of the developed world.
Body proportions of babies and children are markedly different from those in adults. A baby’s head is disproportionately large compared with that of an adult, and its legs are disproportionately short. A baby’s head is about a quarter of its length, but an adult’s head is about one eighth of their height.
Between birth and adulthood, a person’s head just about doubles in size, the trunk triples in length, the arms increase their length by four times, and the legs grow to about five times their original length.