This is a benign tumour arising from the trophoblast which occurs in 1 in 2500 to 1 in 5000 confinements in the UK. The incidence of this tumour varies geographically - rates in the USA are similar to those in the UK; in Hong Kong, the incidence rises to 1 in 600 and in Taiwan, to 1 in 200.
Pathologically, a hydatidiform mole appears as a mass of vesicles that is classically described as a "bunch of grapes." Complete moles are characterised by extensive proliferation of trophoblastic tissue, hydropic villi and absence of foetal vessels. In an incomplete mole, hyperplasia of trophoblastic tissue is less marked, some villi are essentially normal and foetal vessels are present.
Beta-hCG is raised.
About 2% of moles develop into choriocarcinomas.