The etiology of developmental delay.

Developmental delay includes mental retardation (IQ less than 70) and borderline intellectual functioning (IQ 71 to 84). The overall frequency is between 1% and 2% of the child population with most cases in the mild range (IQ 50 to 70). Severe mental retardation is mostly due to genetic or acquired biological causes. Mild retardation is more commonly of acquired causes, and in borderline intellectual functioning, sociocultural factors play a role in addition to biological factors. The main prenatal causes include chromosomal and nonchromosomal genetic conditions, metabolic disorders, and adverse events during gestation, whereas environmental factors (eg, alcohol) more commonly cause mild retardation.