Hydatidiform mole is an abnormal or blighted blastocyst after death of the embryo.
This is followed by an excessive (hyperplasia) proliferation of trophoblast to form a vesicular or polycystic mass called hydatidiform mole.
Actually when the embryo dies, the developing villi could not develop further (e.g., tertiary villi) due to lack of blood supply. They start degenerating and cystic swellings develop in degenerating villi.
Moles secrete higher levels of human chorionic gonadotrophins (HCG).
Most of the hydatidiform moles are Monospermic.