Doctors have found many causes. Here are the most common:
Genetic Conditions: Sometimes disabilities are inherited from parents, errors when the genes combine, or other reasons. Examples of genetic conditions include Down Syndrome, Fragile X Syndrome and Phenylketonuria.
Problems during Pregnancy: Mental Retardation can occur when the baby does not develop inside the mother properly. Problems such as the way the baby's cells divide as it grows, when the women drinks alcohol, or gets an infection such as rubella during the pregnancy.
Problems at birth: If the baby has problems during labor and birth, such as not getting oxygen, they may develop the disability.
Health Problems: Diseases like whooping cough, measles, or meningitis can cause the mental disability. Extreme malnutrition, lack of medical care, or being exposed to positions like lead and mercury can also can defects.
Iodine deficiency is the leading, preventable cause in areas of development where iodine deficiency is endemic.