The symmetrical tonic neck reflex (STNR) is a primitive reflex that normally emerges during the first month of an infant's life and is diminished by the age of 9–10 months.[1] It is a bridging or transitional brainstem reflex that is an important developmental stage and is necessary for a baby to transition from lying on the floor to quadruped crawling or walking.[2] In order to progress beyond this development stage, the baby needs to have been successful in unlinking the automatic movement of the head from the automatic movement of the arms and legs.
The STNR is normally fully developed by 6–8 months and significantly diminished by 2–3 years. If this reflex is retained beyond 2–3 years to such a degree that it "modifies voluntary movement", the child is considered to have "immature and abnormal reflex development", and this can have broad effects on the child's later development.[1]