Biological Development of Infants

Children grow dramatically during infancy. The Kids Health website states that during the first year of life, your baby will grow, on average, about 10 inches. She’ll feel heavier too, tripling her birth weight before her first birthday. Inside her body, eye, brain, muscle and skeletal development allow your child to go from a helpless newborn to a walking, talking toddler 23.


At birth, babies can only focus on items close to their faces. According to the Bausch and Lomb website, your baby can see clearly about 8 to 10 inches from her face. This nearsightedness is due to the fact that the tiny muscles around the eye do not know how to work together. During these early months, help your baby develop her eye muscles through mutual gazing. As the baby focuses on different places on your face, such as your eyes or mouth, she strengthens the muscles needed for binocular vision. Moving toys across your baby’s field of vision also helps to develop the muscles for eye coordination. By 6 months, most babies have mastered the skill of binocular vision.


The first years of life show explosive growth of the brain. According to Zero to Three, The National Center for Infants, Toddlers and Families, your newborn comes into the world with a brain that is about a quarter of the size of an adult brain 3. The University of Maine cites that the newborn brain contains about 100 billion neurons 4. However, most of these neurons are unconnected by neural synapses. An easier way to understand this is that the brain’s hardware is present at birth, but wiring takes place as the child learns and plays. By the age of 2 or 3, the brain matures from about 2,500 synapses per neuron to more than 15,000 synapses per neuron.

Muscle Development

Babies are born with specific reflexive abilities designed to help them survive. For example, the rooting reflex encourages the baby to turn his head and suck when something brushes his cheek. The University of Virginia states that muscle development starts with gross motor control and moves to fine motor control 3. The first step in his gross muscle development allows him to lift and support his head 3. He then strengthens his muscles to allow him roll over, sit with support and ultimately crawl and walk. The development of smaller muscles controlling the hands and face allow him to pick up small items and create a myriad of facial expressions 3.

Skeletal Development

At birth, babies have more bones than adults. According to BBC Science, newborn skeletons have 300 bones, while adults have 206. This is due to the fact that newborn skeletons are still largely cartilage separating pieces of bone. Much of this is necessary for other growth. For example, at birth, the skull is split into five pieces. The Lucille Packard Children’s Hospital points out that this split is necessary for the child’s head to fit through the birth canal. It also acts as expansion points as the baby’s brain expands. As the baby grows, this cartilage ossifies, fusing the bone parts. By about 2, the skull is fully fused.