Hydatidiform mole, light micrograph. Hydatidiform mole is an abnormal form of pregnancy in which a non-viable egg fertilized by a single sperm (without the maternal nucleus) implants in the uterus. The typical hydatid mole is a voluminous mass of hydropic swollen chorionic villi, with oedematous, myxomatous and loose stroma, sometimes with cystic dilations, with the macroscopic appearance of grape-like structures. There are also zones of trophoblast hyperplasia.