Nervous and cardiovascular systems are the first functional organ systems during embryonal development. During the first month differentiation and cell division is going on. The process of formation of new neurons propably stops quite early and does not continue after the birth. The prenatal overall growth of the nervous system continues postnatally, especially during the first 3 years of life. It is characterised by growth of the neurons as well as glial cells and myelinization. The frequency of congenital malformations of the central nervous system (i.e. brain and spinal cord) is 1 : 500 – 1 : 3000. More often it affects female fetuses.
Congenital malformations of the CNS are often present in fetuses with chromosomal aberrations (Edward's or Patau syndromes to name a few), but there is no specific anomaly associated with a certain chromosomal abnormality. Meckel syndrome is a rare example of a single gene disorder characterised by major CNS malformations. Teratogens are important in the etiology as well — radiation, chemical teratogens like isotretinoin or methyl-mercury, alcohol; viral infections. Other teratogenic etiological factors are hypoxic-ischemic episodes and some maternal diseases (diabetes, poor controled phenylketonuria or hypotermia). Amniotic bands cause severe mutilations of the face and head.