[Classification of twins and their ultrasonographic diagnosis].

OBJECTIVE

Systematic classification 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

Classification 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.