Moro Reflex: Why Newborn Babies Startle

Newborn babies have several automatic movements called primitive reflexes. These reflexes help babies adapt to life outside the womb and are a normal part of early development. One of the most noticeable is the Moro reflex, also called the startle reflex.^1,2,3

The Moro reflex can happen when a baby is startled by a loud sound, bright light, sudden movement, or a change in body or head position. It most commonly occurs when a baby feels as if they are falling. You may see your baby throw their arms out, open their hands, and then pull their arms back in. Some babies may also twitch briefly in their sleep when the reflex occurs.

The Moro reflex is usually normal and shows that your baby’s nervous and muscular systems are working as expected.^1,2,3 Your pediatrician will also check for it during newborn visits as part of your baby’s routine developmental assessment. Here’s what triggers the Moro reflex, when it goes away, and how you can help soothe your baby when it happens.

What Is the Moro Reflex?

Newborns are born with certain reflexes. When a baby is startled, and the Moro reflex kicks in, they will throw their arms out to the side (away from their body), open their hands and fingers, and extend their neck, making the head go backward and causing the back to arch.

After this initial phase of the reaction, baby will pull their arms back to their body, clench their fists, put their hands by their chest, and then return them to their sides. Sometimes, their legs will kick away from their body and return. If the legs are involved, baby will return to an almost fetal position before relaxing again (occasionally called the newborn scrunch). Baby may cry loudly when they activate the Moro reflex, but not always.^1,5

This startle reflex occurs in a matter of seconds. The first time you see it, it may startle you, too. Your provider checks newborn reflexes as part of your baby’s routine developmental assessment. While baby is lying on the exam table, you may notice your doctor pulling their arms up from their chest. This action will activate the startle reflex, which is one way the doctor can measure baby’s reaction. The Moro reflex is usually weaker in preterm babies due to their lower muscle tone and slower arm recoil. If the reflex is absent, it could indicate other problems with the baby.¹

Related: Baby Milestones Month by Month

What Triggers the Moro Reflex?

When a baby is startled, you will see the Moro reflex. Ordinarily, it comes from sudden stimuli such as sharp body movements or harsh noises. Older siblings can unintentionally startle a baby as they run and play or bump into baby’s bed or swing. Family pets can sometimes cause it, mostly by accident, with barking and quick movements. Babies can even startle themselves when they have a loud cry. Some describe the reflex as a baby twitching in their sleep because they will have a slight startle reflex while sleeping, not cry, and go back to sleep. It can look like a quick twitch or jump before your baby settles back to sleep.⁵

When Does the Startle Reflex Go Away?

The Moro reflex usually starts to fade around 2 to 3 months of age and is often gone by 6 months. These time frames can differ for some babies if they are premature or have other developmental issues. The startle reflex can be seen in the womb as early as 25 weeks of gestation. Babies can actually startle while in the uterus. That would be one heck of a kick-punch combination for mom to feel!¹

When To Call Your Pediatrician

If the reflex is absent at birth or persists beyond 6 months, this can indicate a problem in the child’s nervous or muscular system or development, and they may need further testing. If baby only seems to have the reflex on one side of the body, this also indicates a concern. Your pediatrician will let you know if there is anything to monitor or if further evaluation is needed. Here are some possible problems with or reasons for no startle reflex:^1,8

• An injury during the birthing process
• A congenital brain malformation
• A lack of oxygen (severe asphyxia) to baby during birth
• A brain bleed or intracranial hemorrhage
• An infection
• A weakness in the muscles
• A diagnosis of cerebral palsy

How To Help When Your Baby Startles

Many parents feel frustrated when the Moro reflex wakes their sleeping baby. While the startle reflex is normal and cannot be entirely prevented, there are a few ways to help your baby feel more secure.

• Swaddle your baby safely for sleep, if they are not showing signs of rolling
• Keep your baby as close to your chest as possible while lowering them
• Try to keep baby’s head just a little above the body so their back will hit first before their head

When putting baby to sleep, it also helps to place them in a quiet, dark, and comfortable area. However, you cannot prevent the Moro reflex from occurring.⁶ If you have any concerns about your baby’s startle reflex, ask your provider.

After baby has been startled, you can comfort them by holding them close and making them feel protected. This also promotes attachment to your baby and a feeling of security for them. If the stimulus that triggered the reflex was a loud noise or bright light, you can help soothe them by moving to a quiet, dimly lit area.^1,7

Related: How to Swaddle a Baby: Step-by-Step Guide for New Parents

The Moro reflex can look startling at first, but it is usually a normal part of newborn development. In many cases, it is a sign that your baby’s nervous system is responding as expected.

As your baby grows, the reflex will gradually fade. In the meantime, a calm sleep space, safe swaddling when appropriate, and gentle comfort can help your baby feel secure. If anything about your baby’s startle reflex worries you, reach out to your pediatrician.