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The growth of the infant from the age of 0-3 months to 3-6 months undergoes various changes. The aspects include physical growth, organ system maturation during this stage, sensory development, as well as social, emotional and cognitive development.
1. Physical growth
Weight: The average newborn weighs 7lb 8oz (3,400 g) at birth. Newborns tend to lose up to 10% of their body weight over the first 5 days of life.1 During the first 3 months of life, infants gain about 0.9 kg (2lb) per month. From 3-6 months, they gain about 0.45 kg (1lb) per month.2
Height: An average newborn is 19 to 21 inches (48 to 53 cm) long at birth.1 During the first 3 months, the infant grows about 2.5 to 5.0 cm (1-2in).2 During the first 6 months, the length increases by an inch per month.1
Head circumference: The average head circumference of a full-term newborn is 13-14 inches and increases rapidly during the first 6 months, the average rise being 0.6 inches per month.1
2. Organ system maturation
Brain growth: The nervous system continues to mature through infancy. By the age of 6 months, the infant’s brain weighs half that of an adult.1

Reflexes: The infant begins exhibiting the rooting reflex i.e. turns his/her head when the cheek is stroked. They begin to react to noises, hold their heads up and make quick and jerky arm movements. They can also turn its head easily to both sides, lift it for 1-2 seconds and follow moving objects. By 4-6 months, the baby can hold his/her chest up in a face down position with weight on forearms, roll from stomach to side, as well as from back to stomach and stand with support.3
Teeth: An infant is usually born with one or more natal teeth or can develop them within the first 28 days of life. However, this might not always be the case.
Digestion: The newborn’s digestive system is not developed completely. Small amounts of saliva can be present in the first 3 months of life. The stomach capacity is relatively small at birth and holds about one-half to 1 ounce. The small intestine is about 250 cm and the liver is immature.1
3. Sensory development
Sight: Newborns can view objects at a distance of 8-15 inches. At one month, they can recognise known faces well. The ability to fuse two ocular images into one picture or binocularity begins to develop at 6 weeks of age and is well established by 4 months.1 By 3-4 months, your baby can take details and build a 3-D picture of the world. By 5-6 months, he/she can discriminate between different facial expressions and respond to them.4
Hearing: The least developed of all senses at birth, most infants can recognise the location of sound within the first 3-4 days of life and can respond to human voices.5
Smell and taste: The cells for these are well developed at birth.5
4. Social, emotional and cognitive development
By 2 months of age, your baby is ready to begin socialising. He/she exhibits a first real smile at 2 months of age and by 3 months, begin interacting with the caregiver and mimic him/her.1
Within half an hour of birth, your baby can flick his/her eyes when spoken to. By 6 months, your baby is good enough to make sounds to attract your attention. By 3-6 months, the baby is so socially sound that he/she can reach for and grasp objects, learn to sit with support and begin to coo and babble.4
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