It is an abnormal condition of the placenta where there are partly degenerative and partly proliferative changes in the young chorionic villi. These result in the formation of clusters of small cysts of varying sizes. Because of its superficial resemblance to hydatid cyst, it is named as hydatidiform mole. It is best regarded as a benign neoplasia of the chorion with malignant potential.
There is wide range of geographical and ethnic variation of the prevalence of the condition. The molar pregnancy is common in original countries- Philippines, china Indonesia, Japan, India, central and latin America and Africa. The highest incidence is in Philippines being 1 in 80 pregnancies and lowest in European countries 1 in 752 and USA Being About 1 In 2000 the incidence, in India, is about 1 in 400.