The heart is the first major organ to develop and function in a growing child. It begins forming just 16 days after fertilization and starts pumping fluid through blood vessels by the fifth week of pregnancy. From that point forward, development follows a remarkably predictable sequence, both before and after birth, with each new ability building on the ones that came before it.
How Organs Develop Before Birth
Development in the womb follows a strict order. The heart and major blood vessels form first, around day 16. By week four, the neural tube (the structure that becomes the brain and spinal cord) starts taking shape. The brain and spinal cord are unique in that they continue forming and refining throughout the entire pregnancy, long after other organs have settled into their basic structure.
By week seven, the lungs, digestive system, and kidneys begin to develop. By the end of week ten, all major organ systems have formed, though they still need months of growth before they can function on their own. The lungs are a good example of this lag: air sacs don’t start forming until weeks 22 to 24, and the lungs may not be mature enough to breathe air until around weeks 25 to 27.
Reflexes Come Before Voluntary Movement
Babies are born with a set of automatic responses called primitive reflexes. These originate in the brainstem and exist purely for survival. The sucking reflex, which helps a newborn coordinate feeding with breathing, first appears at just 14 weeks of gestation. The Moro reflex (a startle response where the baby flings out its arms when it feels like it’s falling) develops by 28 weeks, as does the grasping reflex. The rooting reflex, where a baby turns its mouth toward a touch on its cheek, begins at 32 weeks.
These reflexes are temporary. Between four and six months of age, the brain matures enough to suppress them and replace them with voluntary, intentional movements. A baby who once grasped your finger automatically will start reaching for objects on purpose instead.
Development Moves Top to Bottom, Center to Edge
Two patterns govern the order in which a child gains physical control. The first is the top-to-bottom pattern: babies control their head and neck before their arms, and their arms before their legs. That’s why lifting the head is one of the earliest motor milestones and walking is one of the latest. The second pattern runs from the center of the body outward: infants can move their arms before they can use their fingers with any precision.
These two rules explain the entire sequence of motor milestones. By around two months, most babies can lift their head and chest while lying on their stomach. By four months, they can roll from front to back. By six months, they can sit with little or no support. Each milestone depends on mastering the one before it.
Senses Develop in a Specific Order
A baby’s senses don’t all come online at the same time. Touch is the first sense to develop in the womb, followed by the ability to respond to vibrations and sound. Hearing matures well before vision. Smell is also processed before birth. Vision is the last major sense to develop and continues maturing for months after a baby is born.
This sequence is reflected in how the brain wires itself. Sensory networks for touch, hearing, and vision are functional at or near birth, showing adult-like organization even in premature babies born at full-term age. Higher-order brain networks, the ones responsible for executive functions like planning, decision-making, and impulse control, take much longer. These networks gradually emerge over the first two years of life and keep refining well into adolescence. Synapse numbers in sensory areas of the brain peak years before those in the prefrontal cortex, which handles complex thinking.
Social Skills Before Language
Social and emotional development starts surprisingly early. Around two months of age, babies produce their first “social smile,” a genuine smile directed at another person rather than a random facial movement. Around the same time and through about four months, babies begin forming attachments to their caregivers.
Between six and nine months, this bond deepens. Babies develop a clear preference for familiar people and start recognizing strangers as unfamiliar, which can trigger what’s known as stranger anxiety. By nine to twelve months, most babies show obvious affection toward their preferred caregivers and become distressed when those people leave, a behavior called separation anxiety.
Language develops on a parallel but slightly slower track. Between four and six months, babies start vocalizing vowel sounds, sometimes combined with a consonant (“uuuuuummm,” “aaaaaaagoo,” “daaaaaaaaaa”). Between seven and nine months, they begin babbling long strings of repeated sounds like “mamamama” or “babababa.” So a baby smiles socially months before producing anything resembling a word, and babbles meaningfully before stranger anxiety fully sets in.
The Brain Builds Simple Circuits First
All of these milestones reflect the same underlying principle: the brain builds its simplest, most essential circuits first and layers complexity on top. Sensory and motor networks mature before the networks that interpret and make decisions about that sensory input. A baby can see and hear long before it can understand what it’s seeing or hearing.
This hierarchy matters practically. It means that early sensory experiences, being touched, hearing voices, seeing faces, are literally shaping the foundation that more advanced cognitive skills will be built on. The brain’s higher-order regions don’t develop in isolation. They wire themselves by connecting to the sensory networks that were already in place, building outward in successive steps from sensory areas to regions involved in memory, emotional processing, and eventually complex reasoning.
Putting the Full Sequence Together
Here’s the order in which major developments typically unfold:
- 16 days after fertilization: Heart begins forming
- Week 4: Brain and spinal cord start developing
- Week 7: Lungs, kidneys, and digestive system begin forming
- Week 10: All major organ systems are in place
- Week 14: Sucking reflex appears
- Weeks 25 to 27: Lungs may be mature enough to breathe air
- Birth: Primitive reflexes (Moro, grasping, rooting) are active; sensory networks are functional
- 2 months: Social smile, head lifting
- 4 to 6 months: Rolling over, early vowel sounds, primitive reflexes fade
- 6 months: Sitting with minimal support, solid food readiness
- 6 to 9 months: Stranger anxiety, consonant babbling
- 9 to 12 months: Separation anxiety, strong caregiver preference
The consistent theme across every stage is that basic structures and functions always precede complex ones. The heart beats before the brain thinks. Reflexes precede intentional movement. Sensory processing precedes reasoning. Social connection precedes language. Each layer of development provides the scaffolding for the next.