A hydatid cyst (also known as a mole) is a mass of placenta-derived cells resembling a grape cluster. Complete hydatidiform moles contain only paternal chromosomes but have a 46,XX karyotype. Complete hydatidiform moles (or complete moles, for short) develop into a hydatid cyst. Complete moles contain no normal placenta and no fetus. Ovarian tetranomas are cases with only maternal chromosomes which exist only as benign tumors.
Partial hydatidiform moles (or partial moles, for short) are triploids containing either an extra maternal chromosome set or an extra paternal chromosome set. Cases with an extra set of maternal chromosomes are severely retarded with small, fribrotic placentas. Cases with an extra set of paternal chromosomes have abundant trophoblast ("pre-placental") growth and poor embryonic development.