A complete hydatidiform mole is a disorder of pregnancy (gestational trophoblastic disease) in which a genetically abnormal placenta becomes neoplastic. Complete moles are diploid, but with a diandric (both components paternal) complement. This can occurs when an empty ovum is fertilized by one sperm (monospermy, with duplication) or by two sperm (dispermy). Patients with complete mole typically present in the second trimester with enlargement of the uterus and no decline of β-HCG levels normally seen by week 14. Complete mole has a characteristic "snowstorm" appearance on ultrasound.

Grossly, no fetal parts are identified, and the placenta consists of a mass of tissue with "grape-like", swollen chorionic villi. Microscopically, complete hydatidiform mole displays generalized swollen/large villi with central cistern, marked and circumferential hyperplasia of cytotrophoblasts and syncytiotrophoblasts, and cytologic atypia in cytotrophoblasts and syncytiotrophoblasts. No fetal part or fetal red blood cells are usually identified (except in the rare case of twin gestation with one product of conception being complete mole and the other being a normal product of conception.)

Differential diagnoses include partial mole and hydropic abortus. Complete mole demonstrates absent nuclear staining with p57 in the cytotrophoblasts, while partial mole and hydropic abortus both demonstrate nuclear staining with p57 in the cytotrophoblasts.