Regressive developmental disorders are when a child is normal at birth, and begins to regress mentally and physically months or years after birth, in intelligence and physical abilities; and some children develop seizures. Regressive developmental disorders have been hypothesized as genetic, which cannot be the complete answer, because genetics would lead to the disappearance of regressive developmental disorders.

The named regressive developmental disorders are diagnosed based on variations in symptoms, and are incorporated into a larger category of autism spectrum disorders. Regressive developmental disorders have as many as a dozen sub-categories of named syndromes, including Aicardi Syndrome, Angelman Syndrome, Rett Syndrome, Ring Chromosome 20 Syndrome, or Sturge-Weber Syndrome. The symptoms of regressive developmental disorders and childhood seizure disorders overlap—and named regressive developmental disorders have similar signs and symptoms as congenital toxoplasmosis.