The sight of a newborn’s leg trembling or jittering, particularly when they stretch or are roused, can be startling for any parent. This temporary, involuntary shaking is extremely common in the first weeks of life. These movements, often called “newborn jitters” or physiological tremors, relate to the immaturity of the infant’s developing nervous system. While concerning, this shaking is usually a normal, temporary sign of a healthy baby adjusting to life outside the womb.
Why Newborn Jitters Are Common
The primary reason for these tremors is the developing state of the baby’s central nervous system (CNS). Nerve pathways carrying signals from the brain to the muscles are not yet fully matured. This results in an imbalance between the excitatory and inhibitory signals that control muscle movement, leading to temporary, exaggerated motor responses.
This immaturity makes the baby’s movements appear jerky or twitchy, especially when transitioning between states, such as waking up or being stretched. Up to two-thirds of healthy infants experience some form of fine tremor in their first three days of life. These movements are a sign of ongoing neurological maturation, not a neurological problem.
The startle reflex, also known as the Moro reflex, can also trigger these movements. When stimulated by a loud noise, bright light, or sudden change in position, the baby may extend their arms and legs outward before bringing them back in. This involuntary response is a normal newborn reflex that causes sudden, widespread limb movements resembling a shake or tremor. The Moro reflex generally fades by the time an infant is three to six months old.
How to Tell if the Shaking is Benign
The most practical way to differentiate normal jitters from a more concerning movement is by gently intervening. Benign physiological tremors will immediately stop if the parent gently holds or restrains the affected limb. If the movement ceases when you place your hand on the baby’s shaking leg, it confirms the movement is a jitter and not a seizure.
These common tremors are often brief, lasting only for a few seconds, and are symmetrical, affecting both sides of the body equally. Common triggers include being cold, hungry, or overstimulated. Crying can also create shaking or stiffening of the body, which subsides once the baby is soothed.
Shaking may also be observed when the baby is falling asleep or waking up, a phenomenon called benign neonatal sleep myoclonus. These jerking movements associated with sleep are harmless and are typically outgrown by three months of age. If the movement is easily interrupted by touch, it is a harmless, developmental tremor.
Signs That Require Consulting a Pediatrician
While most leg shaking is benign, certain presentations warrant a medical evaluation. The most significant red flag is a movement that cannot be stopped by gentle restraint or holding the limb. If the shaking continues despite holding the leg, it suggests a different type of neurological event requiring professional assessment.
Parents should seek advice if the shaking persists for longer than a few minutes or occurs frequently without an obvious trigger. Other concerning symptoms include changes in consciousness or responsiveness, such as staring blankly or not making eye contact. Any shaking combined with a change in color, like becoming pale or blue around the lips, or difficulty breathing, should be addressed immediately.
A physician will investigate possible underlying causes, such as metabolic imbalances. Conditions like low blood sugar (hypoglycemia) or low calcium (hypocalcemia) can sometimes trigger persistent tremors. Persistent, rhythmic shaking localized to only one side of the body or accompanied by lethargy or poor feeding also requires a thorough medical workup.