Hydatidiform Mole

A hydatidiform mole is an abnormal gestation in which trophoblastic tissue proliferates and chorionic villi become hydropic. It is part of gestational trophoblastic disease (GTD), a spectrum that includes benign molar pregnancy and malignant gestational trophoblastic neoplasia (GTN). A complete mole is androgenetic and diploid: approximately 90% are 46,XX and most of the remainder are 46,XY. The nuclear genetic material is paternal because an empty ovum is fertilized and the paternal genome duplicates or two sperm contribute genetic material. No identifiable fetal tissue forms. The abnormal trophoblast produces large amounts of hCG, which explains the striking endocrine symptoms and the risk of persistent disease. A partial mole is usually triploid—69,XXX, 69,XXY, or 69,XYY—with one maternal and two paternal chromosome sets. Fetal or embryonic tissue may be present, but the placenta contains focal hydropic villi and the pregnancy is not normal. The presence of fetal tissue therefore does not exclude a molar pregnancy. The risk of progression to GTN is higher after a complete mole, approximately 15–25%, than after a partial mole, which is generally under 5%.