Moro reflex
The Moro reflex is a primitive reflex (an automatic response present from birth): after a sudden loss of support, a baby's arms spread out, then come back in.
Remember
The Moro reflex is a normal primitive protective reflex of early infancy. It fades as upper motor neurone control (control from the cells that carry commands from the brain to the cells supplying muscle) matures, and persistence beyond about 6 months is a concern.
A baby who experiences a sudden loss of support or balance spreads the arms out, as if to catch themselves. This is abduction and extension (the arms spread out), followed by adduction and flexion (the arms come back in towards the body, as if embracing). Crying often goes with it.
The Moro reflex is a primitive reflex: an automatic, built-in response that is present from birth. It is protective, and it is also known as the startle reflex, so an extreme stimulus such as a loud noise can set it off as well as a drop in support. The reflex is generated at brainstem level.
The reflex begins to fade at about 12 weeks and has gone by about 6 months. The reason lies in upper motor neurone control. Upper motor neurones are the cells that carry commands from the brain to the cells supplying muscle. In early infancy they are still maturing, so the reflex is not yet held in check. As the brain matures, these cells suppress the primitive reflex circuits, which allows voluntary, purposeful movement to emerge.
At 3 months the reflex is therefore still expected, and its presence is a sign of normal development. If it is exaggerated, or stays beyond the expected age, it may indicate a neurodevelopmental concern (a concern about how the nervous system is developing).
Key facts
- Trigger
- a sudden loss of support or balance, or a startle.
- Movement
- the arms abduct and extend, then adduct and flex.
- Primitive reflex
- an automatic, built-in response that is present from birth, and the Moro reflex is a protective one.
- Timing
- it begins to fade at about 12 weeks and has gone by about 6 months.
- Why it fades
- maturing upper motor neurone control suppresses the primitive reflex circuits.
- If it persists
- retention or exaggeration beyond the expected age may indicate a neurodevelopmental concern (a concern about how the nervous system is developing).
Exam traps
- Finger flexion in response to gentle pressure on the palm is the palmar grasp reflex, a different primitive reflex, not the Moro reflex, which is a response of the arms to a sudden loss of support.
- A limb response to turning the head, with extension on the face side and flexion on the opposite side, is the asymmetric tonic neck reflex, another primitive reflex, not the Moro reflex, which is triggered by sudden loss of support and involves both arms symmetrically.
- The Moro reflex does not persist throughout adult life as a reflex that maintains balance: it begins to fade at about 12 weeks and has gone by about 6 months.
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