The loss of attained milestone is called as Developmental regression. This is worrisome for parents as this could signal to a serious neurological disorder. Regression could be in the form of loss of walking ability, speech loss or development of cognative impairment.
Developmental regression could be due to metabolic disorders, progressive hydrocephalus, worsening of seizures, increased spasticity, worsening of movement disorders or parental misconception of acquired milestones.