Trembling, Jitters, and the Startle Reflex: What's Normal in a Newborn

The dramatic startle, the trembling chin after a cry, the rhythmic shake of an arm during a nappy change — newborn movements are alarming to watch precisely because they look out of control. Most of them are normal features of an immature nervous system. But there is one specific clinical question worth knowing how to answer at home: does the movement stop when you hold the limb still? If yes, it's benign jitteriness. If no, and the baby seems "elsewhere" or has eye or colour changes, that's a neonatal seizure until proven otherwise and it's a 999 call. Healthbooq gives parents the specific signs to look for in the first weeks, so the calls you make to the GP, midwife, or 999 are the right ones.

The Moro (Startle) Reflex

The Moro is the most theatrical of the newborn reflexes. Triggered by a sudden change in head position, a loud noise, or the sensation of falling, it goes through two phases:

1. Extension: the arms shoot out to the sides, fingers splayed, head extends backward.
2. Flexion: the arms curl back across the chest in an embrace-like motion, often with a cry.

It's a brainstem-mediated reflex and reflects an immature cortex that hasn't yet learned to inhibit it. Present from birth. Disappears between 3 and 6 months as cortical control develops.

What's normal: symmetric response (both arms move the same way), triggered by a clear stimulus, baby settles afterwards.

• Asymmetric Moro — one arm doing less than the other. This can indicate Erb's palsy from a brachial plexus injury at birth, a fractured clavicle, or a neurological problem on that side.
• Persistent Moro past 6 months — should have faded by then.
• Absent Moro at birth — usually picked up at the newborn examination.

Mention asymmetry to the health visitor, GP, or paediatrician at the next contact, sooner if you're noticing it consistently.

Jitteriness: Common, Usually Benign

Neonatal jitteriness is a rapid, rhythmic, symmetrical trembling of the chin, jaw, tongue, or limbs. The frequency is fast — several oscillations per second — and the amplitude is small. It looks like shivering.

It's seen in roughly 40% of healthy term newborns at some point in the first few days. Triggers include handling, undressing for a nappy change, hunger, crying, a cold room, or a loud noise. It's usually most pronounced in the first 24–72 hours and settles over the first 1–4 weeks as the nervous system matures.

The Bedside Test That Tells You What It Is

This is the single most important thing to know:

Gently hold the trembling limb still.

• Jitteriness stops. The movement is stimulus-sensitive and disappears when you suppress it manually.
• A seizure does not stop. The movement keeps going regardless of whether the limb is held.

That one test, combined with looking at the eyes and face (see next section), distinguishes the two in almost every case. Clinicians use the same test.

When Jitteriness Has a Cause Worth Treating

Most jitteriness is benign and self-resolves. A small subset reflects a metabolic or neurological problem:

• Hypoglycaemia (low blood sugar). Higher risk in babies of mothers with diabetes (gestational or pre-existing), small-for-gestational-age babies, large-for-gestational-age babies, premature babies, babies who are cold, and babies who aren't feeding well. Untreated hypoglycaemia can cause neurological injury, so jitteriness paired with poor feeding, sleepiness, low temperature, or a high-risk pregnancy history needs same-day assessment.
• Hypocalcaemia (low calcium). More common in babies of mothers with vitamin D deficiency, in premature babies, and in babies of mothers with poorly controlled diabetes.
• Drug withdrawal. In babies of mothers who used opioids, SSRIs, or certain other medications in pregnancy — usually identified in hospital but worth flagging if you weren't asked.
• Hyperthyroidism, electrolyte imbalance, infection. Rarer.

Persistent, very pronounced jitteriness past the first 2–3 weeks, or jitteriness with any of the above context, warrants a GP or midwife review.

Recognising a Neonatal Seizure

Newborn seizures often don't look like the convulsions seen in older children. They are subtle, and they are frequently missed. Features that should make you think seizure rather than jitteriness:

• Movement does not stop when you hold the limb still.
• Eye deviation — eyes pulled to one side, or repetitive blinking/fluttering of the lids that isn't normal blinking.
• Lip-smacking, sucking or chewing movements when not feeding.
• Cycling or pedalling movements of the legs the baby seems unaware of.
• Sudden colour change — pallor, blueness around the lips (cyanosis), or flushing.
• Brief apnoea — a pause in breathing, often with the other features above.
• Sudden floppiness or sudden stiffening.
• Altered responsiveness — the baby seems "not there" during the episode.

Causes of neonatal seizures include hypoxic-ischaemic encephalopathy from a difficult birth, intracranial bleed, infection (meningitis, sepsis), metabolic disturbance (hypoglycaemia, hypocalcaemia, hyponatraemia), and stroke. They are emergencies. The first hours of investigation and treatment matter.

When to Call the GP, Midwife, or 999

Call 999 (UK) / 911 (US) immediately if a newborn has:

• Rhythmic movements that don't stop when the limb is held
• Eye deviation or rhythmic eye fluttering
• Sudden colour change to blue, grey, or very pale
• Pauses in breathing
• Becomes very floppy or very stiff
• Is unresponsive or "not there"

Same-day GP, midwife, or 111 (UK) / pediatrician (US) for:

• Marked jitteriness paired with poor feeding, sleepiness, or a low temperature
• Jitteriness in a baby with risk factors for hypoglycaemia (maternal diabetes, SGA, LGA, prematurity, cold)
• Asymmetric Moro reflex — one arm consistently doing less than the other
• Jitteriness still very pronounced past 2–3 weeks

Routine GP or health visitor review if:

• Mild jitteriness persists into the second month
• Moro reflex hasn't faded by 6 months
• You're not sure whether what you're seeing is jitteriness or a seizure — record a video on your phone and show the clinician. This is genuinely useful and clinicians ask for it.

A brief phone video of the episode is one of the most valuable things you can bring to a consultation.

Up to about 40% of healthy newborns have visible jitteriness — rapid, rhythmic trembling of the chin, jaw, or limbs — usually most pronounced in the first 72 hours and settling over the first few weeks. The single most useful clinical test parents (and clinicians) can do at home is gentle restraint: hold the trembling limb still. If the movement stops, it's benign jitteriness. If the movement continues despite restraint, especially with eye deviation, lip-smacking, cycling leg movements, colour change, or unresponsiveness, treat it as a possible neonatal seizure and seek emergency assessment. The Moro (startle) reflex — arms flying out, then back across the body, often with a cry — is a separate, normal reflex present from birth that disappears between 3 and 6 months. Asymmetry of the Moro (one arm responding less than the other) warrants assessment for nerve injury or fracture.