Observing a newborn’s movements during sleep, especially leg or arm shaking, often causes parental anxiety. The sudden twitching of a sleeping infant’s leg is common and usually has a simple physiological explanation. Understanding the difference between these ordinary, non-threatening movements and those suggesting a deeper concern is important. This article explains the underlying causes of infant tremors and provides guidance on distinguishing normal movements from those requiring medical attention.
Why Newborn Movements Are Common
Infants exhibit twitching or shaking movements while resting due to the relative immaturity of their nervous system. A baby’s brain and spinal cord are still developing, and the pathways regulating muscle control are not fully established at birth. The protective sheath around nerve fibers, known as myelin, is not yet completely formed, which leads to momentary lapses in motor control.
Infants spend a large portion of their sleep cycle in active sleep, which is the equivalent of the adult rapid eye movement (REM) phase. During this phase, the brain is highly active, and the body experiences frequent, small bursts of muscle activity. These twitches are believed to be instrumental in sensorimotor development, helping the brain map and learn about the limbs and their capabilities.
Benign Neonatal Sleep Myoclonus (BNSM) is the most common form of this movement, involving fast, repetitive muscle jerks in the limbs or torso. BNSM is a normal, self-limiting condition occurring in otherwise healthy infants. These myoclonic jerks occur exclusively when the baby is asleep or drowsy and are not associated with neurological abnormalities.
Another frequent cause of sudden movement is the Moro reflex, often called the startle reflex. This involuntary response is triggered by a sudden noise, a change in position, or even the baby’s own movement. The Moro reflex causes the baby to briefly stiffen, extend their arms and legs, and then quickly pull them back in. This reflex is a normal part of newborn development and typically disappears by three to six months of age as the nervous system matures.
How to Tell the Difference Between a Tremor and a Seizure
Distinguishing between a normal tremor and a seizure can be difficult, but specific criteria help parents differentiate them. Benign movements, such as BNSM, occur almost exclusively when the baby is asleep or drowsy. The key test is whether the movement stops when the baby is disturbed or roused.
A normal myoclonic jerk will instantly stop if you gently touch, reposition, or wake the baby. If the movement continues unchanged after the baby is fully awake, it is less likely to be BNSM and may warrant further investigation, as seizure activity is typically not interrupted by external stimulation.
The nature of the movement itself also provides clues for parents. Benign movements are often brief, isolated twitches or jerks that may not be perfectly rhythmic. A seizure, however, frequently involves sustained, rhythmic, and repetitive muscle contractions that appear more coordinated. Seizures may also involve other features, such as eye rolling, staring, or subtle lip-smacking, which are not typical of benign movements.
While myoclonus can sometimes appear symmetrical, a seizure may affect only one side of the body or involve the entire body. For parents, the simple test of whether the movement stops upon awakening remains the most practical initial differentiator.
When Shaking Indicates a Medical Concern
While most sleeping tremors are harmless, certain signs prompt immediate contact with a healthcare provider. Any shaking or jerking movement that occurs while the baby is fully awake and alert is concerning, as benign sleep myoclonus is confined to the sleep or drowsy state.
Movements lasting longer than ten seconds warrant concern. Although BNSM episodes can cluster, a single, prolonged episode of sustained shaking is atypical for a benign tremor. If episodes become more frequent or intense over time, a medical evaluation is warranted.
Parents should look for accompanying symptoms suggesting a systemic issue. These include lethargy, inability to feed properly, or difficulty breathing during or immediately after the episode. Paleness or a bluish tint to the skin or lips, especially combined with shaking, requires urgent medical attention. Any involuntary movement resulting in a noticeable change in the baby’s facial expression or level of consciousness should be reported to a pediatrician.