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Why Do Newborns Startle?

By Sarah Harding ; Updated June 13, 2017

The startle reflex, also known as the Moro reflex, is characterized by an infant appearing startled and throwing her arms and legs outward as if she is falling. It is a normal reflex that disappears between 3 and 6 months of age, according to the National Institutes of Health's MedlinePlus website. Initially observing a newborn's startle reflex can be surprising to an unsuspecting caregiver. However, it is perfectly normal and harmless.

Characteristics

While every baby is different, the Moro reflex generally appears in the same manner for all newborns. When startled, the baby flings his arms out sideways with his palms up and thumbs flexed. His legs and feet are thrust outwards and tensed. His face may also stiffen, but crying isn't associated with the reflex.

Function

The Moro reflex is described as primitive because it is almost an overreaction to stimulation. As a newborn gains more control over her muscles, she startles less obviously and less frequently. Startling due to a truly intrusive stimulation continues throughout an individual's life.

Triggers

Nearly any stimulation can trigger the startle reflex in a newborn. For example, a loud noise or the motion of being picked up or set down can trigger this reflex. Sometimes the reflex is set off as the newborn drifts off to sleep.

Testing the Reflex

The Moro reflex may be tested by a baby's pediatrician to confirm motor development and fully functioning neurological systems. MedlinePlus suggests that when an infant only exhibits the reflex on one side or not at all, there is a chance that brain, spinal or nerve damage exists. Further medical examination can reveal the cause of the problem. Caregivers and parents should not test the Moro reflex themselves because it could result in an accidental injury, as it involves lifting the infant's head from a soft surface and allowing it to fall back down suddenly.

Considerations

When the Moro reflex persists beyond 6 months of age, you should consult a health-care professional. Startling due to intrusive stimulation is normal for people of all ages, but a true Moro reflex only occurs at birth through the first three to six months of life. It is perfectly normal for an infant to exhibit the same type of startle motions when a loud sound or other stimulation occurs. Infants may cry out or wake in these situations, whereas the Moro reflex makes it appear as if the infant doesn't notice her own bodily movements.

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