The complete hydatidiform mole is usually diploid and
entirely androgenetic in origin. Most have 46,XX karyotype;
a few have a 46,XY karyotype. A complete molar
pregnancy consists of diffuse hydropic chorionic villi
with trophoblastic hyperplasia, forming a mass of multiple
vescicles. There is usually no evidence of a fetus
and minimal embryonal development.
The partial hydatidiform mole is usually triploid, with one
maternal and two paternal haploid sets, either from dispermic
fertilization or from fertilization with an unreduced
diploid sperm. There is usually a fetus and a large
placenta. The hydropic villi show a less florid appearance
than is seen with a complete hydatidiform mole and
are interspersed with normal chorionic villi.