Microcephaly is diagnosed when the head circumference is less than two standard deviations below the head circumference of a child of the same gestational age. Microcephaly is defined as a head circumference that is more than two standard deviations below the mean value for a child of the same gestational age, and is not a specific value. Microcephaly is common in the mentally retarded population, and children with microcephaly may have normal cranial morphology, or they may have significant malformations or enlarged ventricles. Causes can be categorized as primary or secondary, with primary microcephaly including familial, autosomal dominant and a range of chromosomal syndromes. Secondary microcephaly is due to a large number of deleterious factors, such as exposure to radiation, congenital cytomegalovirus, rubella virus infection, or toxoplasmosis before the 15th week of gestation, and can also be seen in meningitis, malnutrition, maternal diabetes mellitus, or hyperphenylalaninemia. If the head circumference is more than two standard deviations below the average for the same sex and age, it is advisable to consult a doctor to find out the cause of the problem, so that early detection and treatment can be carried out to avoid more serious consequences.