Intellectual Disability (Intellectual Developmental Disorder) 39
others (e.g., San Phillippo synd rome) progressive worsening of  intellectual function. After
early childhood, the disorder is generally lifelong, although severity levels may change
over time. The course may be influenced by underlying medical or genetic conditions and
co-occurring conditions (e.g., hearing or visual impairments, epilepsy). Early and ongoing in-
terventions may improve adap tive functioning throughout  childhood and adulthood. In
some cases, these result in significant improve ment of intellectual functioning, such that
the diagnosis of intellectual disability is no lo nger appropriate. Thus, it is common practice
when assessing infants and young children to de lay diagnosis of intellectual disability un-
til after an appropriate course of intervention  is provided. For olde r children and adults,
the extent of support provided may allow for fu ll participation in all activities of daily liv-
ing and improved adaptive function. Diagnost ic assessments must determine whether im-
proved adaptive skills are the result of a st able, generalized new skill acquisition (in which
case the diagnosis of intellectual disability may no longer be appropriate) or whether the
improvement is contingent on the presence of supports and ongoing interventions (in
which case the diagnosis of intellectua l disability may still be appropriate).
Risk and Prognostic Factors 
Genetic and physiological. Prenatal etiologies include genetic syndromes (e.g., se-
quence variations or copy number variants involving one or more genes; chromosomal
disorders), inborn errors of metabolism, brai n malformations, maternal disease (including
placental disease), and environmental influences (e.g., alcohol, other drugs, toxins, terato-
gens). Perinatal causes include a variety of labor and delivery-related events leading to