Most babies stop making jerky, involuntary movements between 3 and 6 months of age, though the timeline varies depending on the type of movement. Newborns are born with an immature nervous system, which means their early arm and leg movements look shaky, trembling, or startled rather than smooth and purposeful. This is completely normal and resolves gradually as the brain matures.
Why Newborns Move This Way
A newborn’s nervous system is still under construction. The protective coating around nerve fibers, which helps electrical signals travel quickly and precisely, isn’t fully developed at birth. Without that insulation, signals from the brain to the muscles arrive unevenly, producing movements that look jerky, trembling, or uncoordinated rather than smooth. Think of it like a dimmer switch that hasn’t been installed yet: the brain can turn muscles on and off, but it can’t fine-tune the intensity or timing.
On top of that, babies are born with a set of primitive reflexes, automatic motor responses that served a protective purpose in the womb and during early life. These reflexes fire involuntarily, which means your baby isn’t choosing to fling their arms out or jerk their legs. Their nervous system is doing it for them.
The Startle Reflex
The most recognizable jerky movement is the Moro reflex, commonly called the startle reflex. When a baby feels a sudden change in position, a loud noise, or any abrupt stimulation, their arms extend outward with fingers splayed, their back arches slightly, and then the arms pull back in toward the body. It looks dramatic, and it often wakes babies from sleep.
The Moro reflex is present in all healthy full-term newborns. It begins to fade around 12 weeks and is typically gone completely by 6 months. If the startle reflex persists well beyond 6 months, pediatricians consider this a possible sign of a neurological issue that warrants evaluation.
The Fencing Reflex
Another reflex that produces unusual-looking movements is the asymmetric tonic neck reflex, sometimes called the fencing reflex. When a baby’s head turns to one side, the arm on that side extends while the opposite arm bends, creating a pose that looks like a fencer’s stance. This reflex is most prominent between 1 and 4 months and disappears somewhere between 3 and 9 months.
Jerky Movements During Sleep
Many parents notice their baby twitching, jerking, or making repetitive movements specifically during sleep. This is often benign neonatal sleep myoclonus, a harmless pattern of muscle jerks that occurs during deep sleep. The twitches usually affect the hands, feet, and limbs, last 10 to 20 seconds at a time, and can sometimes continue in clusters for 30 minutes or longer.
The key feature that separates these sleep jerks from anything worrisome is that they stop immediately when the baby wakes up. If you gently rouse your baby and the movements cease, that’s a reassuring sign. These sleep jerks can start in the first week of life and resolve on their own between 2 and 6 months of age, with normal development afterward.
When Movements Become Smooth and Purposeful
The transition from jerky to smooth movement doesn’t happen overnight. It unfolds gradually over months and even years as the nervous system matures.
Around 4 to 5 months, babies begin coordinating their eye and head movements to smoothly track objects. Reaching for toys starts between about 3 and 6 months, but those early reaches are still crooked and uneven, with the arm speeding up, slowing down, and changing direction several times before making contact. The automatic newborn stepping reflex, where a baby makes walking motions when held upright, typically disappears by 2 months and reappears around 8 to 10 months as supported walking begins.
It actually takes years before a child’s reaching and fine motor movements become as smooth and straight as an adult’s. So while the involuntary jerky reflexes largely resolve by 6 months, the broader shift toward fully coordinated movement is a much longer process.
How Swaddling Helps
If your newborn’s startle reflex is disrupting sleep, swaddling can make a real difference. Wrapping a baby snugly mimics the confined feeling of the womb and physically limits the arm-flinging motion of the Moro reflex. Research consistently shows that swaddled newborns spend more time in deep sleep, experience fewer spontaneous arousals, and sleep for longer stretches compared to unswaddled babies. Even babies who weren’t previously accustomed to swaddling showed increased quiet sleep and fewer awakenings once the technique was introduced.
Swaddling is most useful during the first 3 to 4 months, when the startle reflex is strongest. Once your baby starts showing signs of rolling over, it’s time to transition out of the swaddle for safety.
Normal Jitteriness vs. Concerning Movements
It helps to know what separates normal newborn jitteriness from something that needs medical attention. Normal trembling or jitteriness has three features: it can be triggered by a stimulus like a loud noise or sudden movement, it stops when you gently hold or flex the affected limb, and it doesn’t involve unusual eye movements like the eyes locking to one side.
Infantile spasms, by contrast, look different in important ways. Each spasm lasts one to two seconds and repeats in clusters every 5 to 10 seconds. They often involve sudden stiffening, the body bending forward, arms and legs pulling inward, repeated head nodding, or the eyes rolling upward. These clusters typically happen just after a baby wakes up and rarely occur during sleep. While they can resemble a startle reflex at first glance, the repetitive, clustered pattern is the distinguishing feature. Infantile spasms require prompt medical evaluation.
If your baby’s jerky movements can’t be stopped by gentle restraint, involve forced eye deviation, come with pauses in breathing, or occur in repetitive clusters after waking, those are reasons to contact your pediatrician promptly rather than waiting for the next scheduled visit.