Objective: Systematic classifi cation of all forms of twins and mechanisms of twinning. Design: Review of published cases. Morphogenesis based on personal experience related to human blastogenesis including observation of anterior twinning in two early human embryos. Setting: Institute for the Care of Mother and Child, Prague, Department of Gynecology and Obstetrics, 1st Medical Faculty, Charles University, Prague, Department of Medical Genetics, Medical Faculty, Palacký University Olomouc, Institut Unica, Brno. Methods: Analysis of cases described in literature completed by observed cases. Results: Classifi cation of twins. A) Separated twins: 1. dichorial a) monozygotic (very rare), b) dizygotic (most frequent); 2. monochorial (always monozygotic) a) diamnial, b) monoamnial. B) Conjoined twins (always monozygotic) 1. isopagi (equal conjoined twins) a) originating from peripheral fusions of two germ discs, b) originating from duplications of axial structures; 2. heteropagi (unequal conjoined twins): autosit (main twin), heterosit (parasitic twin). Conclusion: The developmental mechanisms of twinning are discussed, special attention is paid to equal conjoined twins and to the possibilities of their early prenatal ultrasonographic diagnostics.

        Key words: separated twins, conjoined twins, unequal twins, classifi cation, prenatal ultrasonographic diagnostics