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The American Academy of Pediatrics describes it precisely: "If your baby's head shifts position abruptly or falls backward — or if he is startled by something loud or abrupt — he will extend his arms and legs and neck and then rapidly bring his arms together. He may even cry loudly." That is the Moro reflex, and it is the single most alarming-looking normal thing a newborn does.

It is involuntary. The AAP is explicit that most of a newborn's movements in the first weeks "are done by reflex", meaning they happen without your baby trying and without your baby being frightened in the way an older child would be. A startle is not distress. It is wiring.

When it appears and when it goes

The AAP publishes a table of newborn reflexes with expected timings. The Moro reflex appears at birth and disappears at around two months. The AAP adds that it "is present in varying degrees in different babies, usually peaks during the first month and starts to disappear after about two months."

For context, the same table gives:

Stepping — birth to 2 months.
Rooting — birth to 4 months.
Palmar grasp — birth to 5 to 6 months.
Tonic neck (the "fencing" posture) — birth to 5 to 7 months.
Plantar grasp — birth to 9 to 12 months.

The AAP notes that "not all infants acquire and lose these reflexes at exactly the same time", so a baby still startling at ten weeks is not late. The reflex fades rather than switching off.

Why loud noises are not the only trigger

The trigger that surprises parents most is not sound at all. It is the sensation of falling. A head that drops backward as you lower your baby into a cot sets the reflex off just as reliably as a slammed door, which is why the startle so often happens at the exact moment you put a sleeping baby down.

The AAP's account of newborn behaviour puts the noises in context too: newborns produce "a wide variety of grunts, squeaks, sighs, sneezes, and hiccups", and many of these "just like their sudden movements, are reactions to disturbances around them; a shrill sound or a strong odor may be all it takes to make them jump or cry."

The one thing worth checking

The AAP gives a single, specific instruction about this reflex, and it is the reason this page exists: "Both the Moro and tonic neck reflexes should be present equally on both sides of the body. If you note that the reflex seems different on one side, or that the baby moves one side of the body better than the other, tell your pediatrician."

So the question is not how much your baby startles. It is whether both arms do the same thing. An arm that consistently does less than the other is worth mentioning — the AAP notes elsewhere that a stretched nerve or a broken collarbone during birth can leave one arm or shoulder weaker for several weeks, and these usually recover, but they should be looked at rather than assumed.

What helps at night

Nothing stops the reflex, and nothing should. What you can do is reduce the falling sensation.

Lower your baby bottom first, head last, keeping their head supported until it is resting on the mattress.
Keep contact for a moment after you put them down, with a hand on the chest, before you move away.
Swaddling holds the arms in and does reduce startle waking. The Lullaby Trust's safer sleep guidance is that if you swaddle, use thin materials, never cover the head, do not swaddle above the shoulders, and always place your baby on their back — and that swaddling should stop once your baby can roll or shows signs of being able to.
Do not overheat. The Lullaby Trust is clear that overheating raises the risk of sudden infant death, and a swaddled baby needs fewer layers, not more.

The HSE's advice on soothing a newborn is worth borrowing here: skin-to-skin contact, holding your baby upright against you, gentle movement and your voice are the things that reliably settle a startled newborn. There is no equipment that does this better than you do.

When to get help

Call your local emergency number (999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand) if your baby:

Has a seizure, or jerking or twitching movements that carry on when you hold the limb still.
Looks pale, ashen, mottled or blue.
Is unresponsive or cannot be roused, or has a weak, abnormally high-pitched or continuous cry.
Is grunting with each breath, breathing more than 60 breaths a minute, or drawing their chest in under the ribs.
Has a temperature of 38C (100.4F) or above, or below 36C (96.8F).
Has a bulging fontanelle, a non-blanching rash, or neck stiffness.

That list is NICE's own set of red flags for serious illness in young babies, published in its postnatal care guideline NG194. None of them are the Moro reflex. A startle is brief, it is symmetrical, and your baby settles afterwards.

Speak to your midwife, health visitor, public health nurse or GP if: the reflex looks different on one side, one arm or leg moves less than the other, your baby seems floppy or stiff between startles, or your baby has stopped startling to loud noise by around a month old — that last one is a hearing question, not a reflex question, and is worth raising.

What to let go of

A baby who startles a lot is not anxious, badly handled or being spoiled by noise. No health body advises tiptoeing around a newborn; the NHS's own advice on the early days is the opposite, that babies get used to normal household sound. There is nothing to train, no product to buy, and no reason to keep the house silent. The reflex leaves on its own, usually before your baby is old enough to notice it ever existed.

Newborn Startle Reflex