Gestational trophoblastic tumours group together the complete hydatidiform mole (classical) and partial mole (with fetus), invasive mole and choriocarcinoma. Genetically, trophoblastic tumours can arise in different ways: they can derive from normal zygotes with the maternal and paternal haplotype (normal pregnancy), or from a triploid zygote (partial mole) or from an XX zygote possessing only a duplicated male haplotype and no maternal contribution (complete mole). The low malignancy rate in partial mole (2.4%) compared to the complete mole (10 to 20%) remains unknown. Further analysis of the genetics of these tumours may well contribute to the understanding of the process of carcinogenesis.