Parents often observe their newborn suddenly “jump” or startle while sleeping, often causing the baby to wake up. This sudden, jerky motion can look alarming, but it is a universal and expected part of early infancy. The movements are a response to an immature nervous system reacting to the world outside the womb. Understanding the biological basis of these movements helps parents manage them and ensure the baby is comfortable.
The Moro Reflex and Newborn Development
The sudden, involuntary movement that looks like a baby is jumping is formally known as the Moro reflex, or the startle reflex. This is a primitive reflex, meaning it is an automatic, non-learned response present at birth. The reflex serves as a protective mechanism and indicates healthy neurological function.
This reflex is typically a two-phase reaction that occurs when a newborn is exposed to a sudden environmental change. Common triggers include a loud noise, a bright light, a quick change in head position, or the sensation of falling when being placed into a bassinet.
The first phase involves the baby quickly extending their arms outward and opening their hands, often accompanied by a gasp. The second phase is drawing the limbs back in toward the body in a flexed, embracing posture. The reflex is most prominent in the first month of life. It begins to diminish as the baby’s brain matures, typically fading away completely between two and six months of age.
Identifying Normal Movements
While the Moro reflex is the most noticeable startle, newborns exhibit other normal, non-concerning movements during sleep. Benign Neonatal Sleep Myoclonus (BNSM) involves brief, repetitive twitching or jerking of the arms, legs, or trunk that only occurs during quiet sleep. These movements are harmless, not associated with seizures, and stop immediately if the baby is gently roused or wakes up.
A normal Moro reflex is symmetrical, meaning both sides of the body react equally, and it is a single, quick event that resolves rapidly. Movements that are concerning warrant a call to your pediatrician. These include movements that are rhythmic, repetitive, and sustained for more than a few seconds, or those that are asymmetrical, affecting only one side of the body. Unlike the Moro reflex, problematic movements often continue even when you touch or reposition the baby, or occur when the baby is wide awake and alert. It is helpful to record a video of the movement to show your doctor.
Techniques for Reducing Startles
Since the Moro reflex can disrupt sleep, parents can employ several strategies to help mitigate its impact. The most common and effective technique is swaddling, which involves wrapping the baby snugly in a thin, breathable blanket. Swaddling recreates the secure, confining feeling of the womb and physically restricts the arm movements that trigger the startle reflex.
When swaddling, ensure the wrap is snug around the chest but allows for proper hip development. The baby’s legs must be able to bend up and out into a frog-like position. Swaddling should be discontinued immediately once a baby shows any signs of starting to roll over, typically around two to four months of age, as it becomes a suffocation risk.
A simple but effective strategy to reduce the startle when laying a baby down is to use the “waking up slowly” technique. As you lower your baby into the crib or bassinet, keep them close to your body for as long as possible to prevent the sensation of free-falling. Once their back touches the mattress, keep your hands gently on them for a few moments before slowly withdrawing contact.
Maintaining a calm sleep environment can also help to avoid triggering the reflex. Using a white noise machine can mask sudden, jarring sounds, such as a door closing or a dog barking. Keeping the room dimly lit also reduces the likelihood of the reflex being triggered by sudden changes in light intensity.