Heron Werner, MD & Pedro Daltro, MD
ClÃnica de Diagnóstico por Imagem (CDPI) & Instituto Fernandes Figueira (IFF) – FIOCRUZRio de Janeiro – Brazil
Dorothy I. Bulas M.D.
Professor of Radiology and Pediatrics
Children"s National Medical Center
George Washington University Medical Center
111 Michigan Ave, NW,  Washington D.C. 20010
Conjoined twins results from the division of the embryonic disc at least 13 days after fertilization. They must always be monozygotic, monochorionic and monoamniotic. The exact incidence of conjoined twins is not known, but estimates have varied from 0.0125-0.04:10,000 births. They are easily diagnosed by ultrasound, especially after the first trimester. Nevertheless, MRI can helps their evaluation in the third trimester, which allows assessment of prognosis, surgical planning and patient counseling before the delivery. We should suspect of conjoined twins when no amniotic membrane is identified by ultrasound or when the twins face each other. Thoracopagus is the most common type and with omphalopagus or xiphopagus, represent about 75% of cases. Nevertheless, there are others less common sites of junction including cephalic (craniopagus) or caudal (ischiopagus).