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, and a boy is 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 trebles in length, the arms increase their length by four times, and the legs grow to about five times their original length.
At birth, babies have almost no ability to control their movements. At the age of about four weeks, a baby placed on its stomach can usually hold its head up. At about four months, the baby will usually be able to sit up with support, and at the age of seven months should be able to sit alone. At around eight months, most babies can stand with assistance, and will start to crawl at ten months. They can probably put one leg after the other if they are led at about 11 months, and pull themselves up on the furniture by one year. At about 14 months a baby can usually stand alone, and the major milestone of walking will probably occur around 15 months.
These are average figures and many children will reach them much earlier and others much later. Physical development does not equate with mental development, and parents should not be concerned if their child takes its time about reaching the various stages - Einstein was so slow in learning to talk that his parents feared he was retarded.
Most newborn babies sleep most of the time - although there are wide variations and some babies seem to stay awake most of the day and night, to the distress of their parents. As they grow, a baby's need for sleep diminishes until a toddler requires about ten or twelve hours of sleep a night, with a nap in the daytime.