Hydatidiform moles are intriguing pathologic entities representing abnormal placental villous tissue with unique genetic profiles and a wide spectrum of morphologic features, which makes accurate diagnosis challenging. Overrepresentation of the paternal genome in sporadic hydatidiform moles (purely androgenetic in complete hydatidiform moles and diandric triploid in partial hydatidiform moles) is a fundamental genetic event leading to global alteration of imprinting gene expression in the molar trophoblast. Rare familial biparental hydatidiform moles (due to NLRP7 or KHDC3L mutations) share such global imprinting alterations, implying a common end point of pathogenesis. Despite being the cornerstone of diagnosis, routine morphologic assessment of hydatidiform moles continues to suffer from interobserver diagnostic variability, emphasizing the need for new diagnostic modalities. Analyses of p57 expression by immunohistochemistry and polymerase chain reaction–based DNA genotyping have emerged as powerful diagnostic methods for accurate classification of hydatidiform moles. Algorithmic approaches combining histology and these ancillary techniques provide the best diagnostic practice currently available.

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