Neuroscientists say about 90 percent of brain development occurs by age five. The process starts in utero, and while it does continue into adulthood, the brain develops at a much faster rate through the first 1,000 days of life than at any other time. Those early years are when the brain is the most “plastic,” meaning it has the greatest capacity to observe, adapt, and learn new skills and abilities, from recognizing parents’ faces to chucking Cheerios to talking and walking.
But not all kids’ brains progress at the same pace or in the same way. Brain development is driven by a mix of genetics, nutrition starting in utero, and a child’s early environments and interactions with people. Exposure to toxins, infections, or chronic stress — either in the womb or post-birth — can influence brain development too, and typically not in a good way.
How the brain develops in those early years lays the foundation for future learning, behavior, and relationships with other people. This is a big reason why it’s important for pregnant people to eat healthy foods, get plenty of rest, and try to mitigate stress — and then, once the child is born, for parents to provide a nurturing, secure environment and offer an age-appropriate, nutrient-rich diet.
We are all born with most of the brain cells we’ll ever have. And physically, a newborn’s brain looks fairly similar to an adult’s brain. “Most structures get bigger as the brain grows, but it’s not the case that one part of the brain is proportionately much smaller when we’re born,” says Elizabeth Norton, PhD, director of the Language, Education and Reading Neuroscience Lab at Northwestern University.
What drives brain development, then, are the millions of neural connections that are made between brain cells and brain regions as babies grow into little kids and eventually into big kids. These connections, which start simple and get increasingly complex, dictate the skills and abilities we acquire at various stages of life, as do a wide range of biological processes that help build brain circuitry.
Obviously, a parent can’t see what’s happening inside their child’s brain to know whether or not it’s developing properly. Instead, their best bet for benchmarking is to look for developmental milestones, such as when their baby learns to smile or when their toddler starts speaking in sentences. Milestones such as these are essentially manifestations of new connections being made within the developing brain.
But, Norton cautions, milestones aren’t a perfect science. She says it’s difficult to ascribe any particular milestone to a singular part of neurodevelopment. Rather, it’s the cumulation of changes occurring within the brain. The time at which certain biological processes peak determines when a child will start laughing, learn language, or begin to read.
Parents should also keep in mind that the age at which kids meet specific milestones can vary from child to child. They can even vary among two kids with the same genes, or two kids with different genes but living in the same environment. “If the kid down the block shows a milestone and yours doesn’t, that doesn’t mean you’re necessarily doing anything wrong or that your child’s brain isn’t developing as well as that child’s,” Norton says.
Brain Stage: In the Womb
What’s going on: Among the many processes that happen in utero, the two major ones are brain cell creation and neuronal migration. “Once brain cells are created, one of their main jobs is to form a brain that functions as well as possible,” Norton says. “They do this by neuronal migration, which means moving to the parts of the brain where they are designed to fit. That could be deep in the hippocampus, where we store memories, or in the part of motor cortex, which helps us move our left arm.”
Because neuronal migration occurs in the womb, it’s largely steered by genetics. “There is an idea that many disorders with genetic basis may be acting on neuronal migration in utero,” Norton says. “For example, the genes associated with dyslexia may affect how those neurons migrate, meaning how the brain is shaped before birth makes someone either better or worse suited to be a good reader.”
Milestones: Babies begin developing their motor and sensory systems in utero. As for the senses, touch typically comes online first, as early as eight weeks gestation. By around 11 weeks, they begin using their hands and feet to feel out their environment and their own bodies. They also respond to their mothers’ movements, sometimes by kicking back.
Babies’ sense of hearing also kicks in early. By about 20 weeks of gestation, their ears are fairly well developed. Starting around week 26 or 27, they can respond to sounds and vibrations such as their mother’s heartbeat or, say, an ultrasound applied to the belly. As time goes on, babies may start recognizing and responding to their parents’ voices.
“Babies are born being able to hear — in fact, the auditory system is almost adultlike at birth,” Norton says. “We know they hear in utero because if a baby is minutes or hours old and you play them speech in a language that’s rhythmically similar to the language they heard in utero, they’ll recognize it. There are also studies showing that within the first couple of days, babies can tell their mother’s voice from another speaker’s voice.”
Eyesight begins developing during pregnancy too, though not nearly as completely as hearing. “We estimate that at birth, babies’ visual acuity is 20/200 or worse, so everything is a bit blurry,” Norton says. “However, if you show them one picture of a [correct] human face and another with parts of the face scrambled, like the eyes on the bottom and the nose on the side, babies take more interest in the picture that looks more like a face.”
Brain Stage: Birth to 12 months
What’s going on: Once a child is born, Norton says the developmental processes occur continuously versus as clear steps. “After birth and through the first few years, there are three main processes going on, all in a continuum,” she says. “It’s not like one process stops and another starts — they are waves of processes that peak at different times.”
One such process is neurons making new connections with one another. “This helps wire together different parts of the brain that need to work together and communicate effectively,” Norton says. One way brain cells do this is by growing more dendrites, which are essentially “arms” that reach out and connect with other brain cells.
The second process is pruning. “Early on, the brain makes extra cells and connections ‘just in case,’ to allow for flexibility where needed,” Norton says. “Then it finds redundancies or connections it doesn’t really need and pulls back on them in order to focus on the ones it does need.”
The third big process is myelination, or white matter development, which Norton says occurs through our twenties and even beyond. “Neurons that get used a lot become wrapped in a little coating of white matter, like electrical tape, that helps messages travel faster and more efficiently,” she explains.
According to Norton, neural connectivity, pruning, and myelination each start in different sequences in different parts of the brain, beginning with the sensory and motor systems, continuing the developments that started in utero. “When we’re first born, we don’t need to do complex social-cognitive thinking like we might in middle school, when we think about things like who is more or less popular than us,” she says. “Our first job is to figure out the environment we’re in and learn how to interact with it.”
One of these jobs is learning language. During the first year of life, Norton says babies experience a sensitive period — a time at which the brain expects or reacts most strongly to certain information — making language learning as easy as possible. “The brain is linking auditory information and cognition and social information to learn language,” she says. “Babies start to realize all the people around them are speaking a language, so they pay more attention to it and take it all in.”
Milestones: From birth, babies start maturing quickly. Because of where in the brain neural connectivity, pruning, and myelination kick in first, the first big milestones parents recognize are in the sensory-motor domains.
In the first three months, most newborns go from wobbly-headed to being able to lift their head and chest when lying face down, according to the Mayo Clinic. They also learn to smile and grasp items with their hands. Their vision gets tuned up too, allowing them to focus on close-up faces, recognize faces from farther out, and follow moving objects with their eyes.
During the four- to six-month stage, babies generally begin to raise their arms, put weight on their limbs, propel themselves, and eventually sit up if helped to the sitting position. They’ll start grasping more objects and sticking them in their mouths, and they’ll begin distinguishing colors and patterns. Babies in this age group may begin babbling and sensing different emotions from different voice tones.
By nine months, babies often can roll over without trouble, sit up or even stand up without much or any help, and begin scooting or crawling. Their dexterity improves considerably, helping them to transfer objects from one hand to the other or into the mouth and even hold utensils. Babies’ communication skills strengthen too. They’ll use sounds, gestures, and facial expressions to speak their minds, and their babbling starts making a bit more sense. Also, because they now recognize family members, they tend to get anxious around strangers.
By about the one-year mark, along with continued sensory and motor refinements, babies will have come a long way in their understanding and expression of language. They can respond to requests, utter words (like mama and dada!), and start being less able to learn languages they haven’t heard before, Norton says. At the same time, their cognition improves significantly, and they often imitate people around them in attempt to learn how to do things on their own.
Brain Stage: 1 to 3 Years
What’s going on: Aside from further development of the sensory and motor systems and cognitive functions, complex brain systems begin interacting more around preschool age. “As the brain grows, we go from big changes in separate systems, such as just in the visual system or just the cognitive system, to linking up different regions and having them work together more efficiently,” Norton says. “We see development in brain regions of the brain that support emotional processing, logic, and reasoning. This is where we get ‘Tommy didn’t share his toy, so I’m not going to let him use my toy.’”
Milestones: During the first few years, kids learn to walk, kick, climb, draw, and all kinds of other physical movements, as well as speak in short sentences. The coalescing of disparate brain systems helps them to follow instructions, have basic conversations, categorize objects, point to objects in picture books, get excited around other kids, and gain independence. Norton adds that preschool-aged kids can also recognize what someone intends to do.
What’s Next: 4 to 6 Years
What’s going on: The fusion between brain regions continues — as do pruning and myelination — enabling kids to learn increasingly complex concepts and skills. A big one is how to read. Interestingly, Norton says that from an evolutionary standpoint, reading is quite new, so there is nothing in our DNA that’s specifically designed to help us read.
“When we learn to read, we are essentially taking areas of the brain associated with visual processing, originally for purposes of things like finding tigers in the jungle, and linking those to spoken language and printed symbols,” she says. “So, at age four and beyond, we learn to read because we link language and visual and cognitive processing regions together more efficiently.” Two-year-olds, by contrast, can’t do that, which is why they’re not yet ready to learn to read.
Milestones: As mentioned, kids in this age bracket typically start reading. They can also count, rhyme, identify colors, draw distinguishable pictures, focus on tasks, recognize familiar environments and transition into new ones, remain calm amidst unexpected change, and play well with other kids.
Again, these, like all milestones, are not set in stone, so parents shouldn’t freak out if their child’s markers don’t align exactly with the age at which most kids experience them. Your pediatrician can help determine if a missed milestone is a cause for concern.