[Epidemiology of identical twin pregnancy].

On the basis of MEDLINE and manual search, we looked at the main papers in English literature published from 1976, regarding risk factors for spontaneous (i.e. not related to fertility drug use) multiple births. The constant frequency of monozygotic (MZ) pregnancies over time and in different geographic areas suggests that determinants of MZ twins are largely unchanged over time and that genetic mechanism may act a role. On the contrary, temporal and geographic trends observed in dizygotic (DZ) pregnancies suggest that environmental factors play a role in determining this condition. At present maternal age and hereditary components are the better defined determinants for spontaneous multiple births.