Seeing a newborn baby shake, twitch, or tremble during sleep can be unsettling for any parent. While the immediate thought often jumps to a serious medical issue, these movements are extremely common in the first few months of life. This phenomenon is usually a harmless display of a rapidly developing nervous system. Understanding the difference between normal movements and those that signal a need for medical attention provides reassurance.
Understanding Normal Newborn Movements During Sleep
The most common movement that alarms parents is a sudden jerk or twitch, often in the arms, legs, or torso. These isolated muscle spasms are known as myoclonus. Benign neonatal sleep myoclonus occurs only when the baby is asleep and is harmless. These movements are typically fast, repetitive jerks that may occur in clusters, sometimes lasting up to 15 minutes.
The movements often involve both sides of the body, appearing symmetrical, but they can sometimes be seen in just one limb. A crucial feature is that these jerks stop immediately when the baby is gently roused or fully wakes up. This myoclonus usually begins within the first few weeks of life, often peaking between two and five weeks. Most babies outgrow these movements by three to six months.
Another benign movement is general jitteriness or fine tremors, seen in the chin or limbs. These tremors are high-frequency and low-amplitude, meaning they are fast but small. They are often triggered by external stimuli, like being unwrapped, a sudden loud noise, or crying. Gently holding the affected limb or providing a pacifier will stop the shaking entirely.
The Underlying Causes of Benign Tremors
The primary reason a newborn exhibits these erratic movements is the immaturity of their central nervous system (CNS). The brain and nerve pathways are still developing and lack the precise control seen in older children. Newborn nerve fibers lack myelin, a fatty protective sheath that helps signals travel smoothly. This incomplete myelination leads to disorganized electrical signals, manifesting as twitches and tremors.
Newborns spend about 50% of their total sleep time in active sleep, also known as Rapid Eye Movement (REM) sleep. During this stage, the brain is highly active, leading to increased muscle activity and more frequent movement, including sudden jerks and twitches. These movements are a normal part of the brain’s developmental process, potentially helping to form motor pathways.
The Moro reflex, or startle reflex, is another developmental factor contributing to movement during sleep transitions. This involuntary reaction to a perceived loss of support or a sudden environmental change causes the baby to suddenly extend their arms and legs outward. This can look like a generalized shake or jerk while they are lightly sleeping. The reflex naturally subsides as the nervous system matures, typically resolving between three and six months.
Distinguishing Harmless Shakes from Medical Concerns
Differentiating between a harmless sleep twitch and a concerning movement like a seizure centers on observation and interaction. Benign tremors and myoclonus only occur during sleep or when highly stimulated, stopping immediately upon waking or gentle restraint of the limb. If the movement continues unabated after you gently touch the limb or fully wake the baby, it suggests the movement is not benign and requires medical evaluation.
Concerning movements are often rhythmic, repetitive, and involve other body systems, occurring when the baby is both asleep and awake. Warning signs include sustained stiffening of the body or limbs, or those accompanied by abnormal eye movements, such as sustained eye deviation or repetitive blinking. Other red flags include changes in breathing, like long pauses or turning blue, or unusual oral movements such as repetitive sucking or smacking.
While most tremors are harmless, some can signal a metabolic imbalance, such as low blood sugar (hypoglycemia) or low calcium (hypocalcemia). If the movements are accompanied by fever, poor feeding, or lethargy, or if they increase in frequency or intensity past the first few months, seeking immediate medical advice is warranted. Recording a video of the event to show a pediatrician is an effective way to help distinguish a normal developmental movement from a medical concern.