Mental retardation is not seen in -
Prenatal factors
1. Metabolic
i) Aminoaciduria-phenylketonuria, homocystinemia, histidinemia, organic aciduria
ii) Galactosemia
iii) Mucopolysaccharidosis
iv)Inherited degenerative disorders of the central nervous system
2. Chromosomal disorders. Down's syndrome, Fragile X syndrome, Klinefelter
3. Environment and nutrition gaps, iodine deficiency
4. Neuroectodermal dysplasia. Tuberous sclerosis.
5. Developmental defects. Microcephaly, craniostenosis, cretinism, porencephaly.
6. Maternal factors
i) Use of teratogenic drugs in the first trimesters of pregnancy
ii) Infections such as rubella, toxoplasmosis, cytomegalic inclusion disease, herpes syphilis and HIV.
iii) Placental insufficiency
iv)Toxemia of pregnancy
v) Antepartum hemorrhage
vi)Radiation during pregnancy
Natal factors

Birth injuries   3. Intracerebral hemorrhage
Hypoxic, ischemic encephalopathy

Postnatal factors

Infections of central nervous system. 

Meningoencephalitis
2. Head injuries
3. Hypoxia
4. Cerebrovascular. Thrombosis of cerebral vessels
5. Post-vaccinal encephalopathies
6. Kemicterus
7. Hypoglycemia
8. Malnutrition
9. Child abuse 
10. Autism
Downs syndrome
Cretinism
Hypopituitarism
Birth asphyxia