In infancy, toddlerhood, and early childhood, the body's physical development is rapid.

On average, newborns weigh between 2.268 kg and 4.535 kg, and a newborn's weight typically doubles in six months and triples in one year. By 2 years of age a child's weight will have quadrupled, from around 9 to 18 kg.

The average length of a newborn is 50 cm, increasing to 75 cm by 12 months and then 87 cm by 2 years of age.

Growth rate

During infancy and childhood, growth does not occur at a steady rate. Growth slows between 4 and 6 years of age. During this time children gain 2 to 3 kg and grow about 5 to 7 cm per year.

Once girls reach 8 to 9 years of age, their growth rate outpaces that of boys due to a pubertal growth spurt. This growth spurt continues until around 12 years of age, coinciding with the start of the menstrual cycle. By 10 years of age, the average girl weighs 39 kg, and the average boy weighs 38 kg.

We are born with all of the brain cells that we will ever have—about 100–200 billion neurons (nerve cells) whose function is to store and transmit information. However, the nervous system continues to grow and develop. Each neural pathway forms thousands of new connections during infancy and toddlerhood. This period of rapid neural growth is called blooming.

Neural pathways continue to develop through puberty. The blooming period of neural growth is then followed by a period of pruning, where neural connections are reduced. It is thought that pruning causes the brain to function more efficiently, allowing for mastery of more complex skills.

Blooming occurs during the first few years of life, and pruning continues through childhood and into adolescence in various areas of the brain.

Brain development

The size of our brains increases rapidly. For example, the brain of a child that is 2 years of age is 55% of its adult size, and by 6 years of age the brain is about 90% of its adult size. During early childhood (ages 3 to 6), the frontal lobes grow rapidly, which are associated with planning, reasoning, memory, and impulse control.

Therefore, by the time children reach school age, they are developmentally capable of controlling their attention and behavior. Through the elementary school years, the frontal, temporal, occipital, and parietal lobes all grow in size. The brain growth spurts experienced in childhood tend to follow Piaget's sequence of cognitive development, so that significant changes in neural functioning account for cognitive advances.

Consider revisiting, for a refresher on Piaget's theory that unfolds in four stages: sensorimotor, preoperational, concrete operational, and formal operational.Motor development

Motor development occurs in an orderly sequence as infants move from reflexive reactions (e.g., sucking and rooting) to more advanced motor functioning. For instance, babies first learn to hold their heads up, then to sit with assistance, and then to sit unassisted, followed later by crawling and then walking.

Motor skills refer to our ability to move our bodies and manipulate objects. Fine motor skills focus on the muscles in our fingers, toes, and eyes, and enable coordination of small actions (e.g., grasping a toy, writing with a pencil, and using a spoon). Gross motor skills focus on large muscle groups that control our arms and legs and involve larger movements (e.g., balancing, running, and jumping).

As motor skills develop, there are certain developmental milestones that young children should achieve. For each milestone there is an average age, as well as a range of ages in which the milestone should be reached.

The following table summarises development milestones for children between 2 and 5 years of age.

Age (years)PhysicalPersonal/SocialLanguageCognitive2Kicks a ball; walks up and down stairs.Plays alongside other children; copies adults.

Points to objects when named; puts 2 to 4 words together in a sentence.Sorts shapes and colors; follows 2-step instructions.3Climbs and runs; pedals tricycle.Takes turns; expresses many emotions; dresses self.

Names familiar things; uses pronouns.Plays make believe; works toys with parts (levers, handles).4Catches balls; uses scissors.Prefers social play to solo play; knows likes and interests.

Knows songs and rhymes by memory.Names colors and numbers; begins writing letters.5Hops and swings; uses fork and spoon.Distinguishes real from pretend; likes to please friends.Speaks clearly; uses full sentences.Counts to 10 or higher; prints some letters and copies basic shapes.

An example of a developmental milestone is sitting. On average, most babies sit alone at 7 months of age. Sitting involves both coordination and muscle strength, and 90% of babies achieve this milestone between 5 and 9 months of age. In another example, babies on average are able to hold up their head at 6 weeks of age, and 90% of babies achieve this between 3 weeks and 4 months of age.

If a baby is not holding up its head by 4 months of age, the child is showing a delay. If the child is displaying delays on several milestones, that is reason for concern, and the parent or caregiver should discuss this with the child's pediatrician. Some developmental delays can be identified and addressed through early intervention.