A complete hydatidiform mole (CHM) coexisting with a viable fetus is a rare finding in pregnancies. Accurate diagnosis often relies on ultrasonographic, histopathological and molecular techniques in the definite diagnosis. To the best of our knowledge, a liveborn fetus coexisting with CHM with trisomy 9 has not been described. The use of molecular genotyping and immunohistochemical laboratory investigations enabled the CHM to be fully characterized. Postzygotic diploidization of a triploid conception arising from dispermy is the proposed mechanism of its formation.