How to deal with a rare entity: the coexistence of a complete mole and a healthy egg in a twin pregnancy?

The association of a normal and a molar egg within a twin pregnancy is extremely rare. The key to diagnosis is the fetal karyotype, thus allowing elimination of its principal differential diagnosis: partial triploid mole. We report a case where the evolution of the pregnancy was complicated by severe toxemia. Interruption of pregnancy was then necessary, even though a conservative attitude had first been considered. Throughout this case, we discuss the means of diagnosis and the clinical handling of this rare entity.