Lissencephaly is a set of rare brain disorders whereby the whole or parts of the surface of the brain are smooth. It is caused by defective neuronal migration during the 12th to 24th weeks of gestation, resulting in a lack of development of brain folds (gyri) and grooves (sulci). It is a form of cephalic disorder. Terms such as agyria and pachygyria are used to describe the appearance of the surface of the brain.