a slow-growing tumor that develops from trophoblastic cells (cells that help an embryo attach to the uterus and help form the placenta) after fertilization of an egg by a sperm. a hydatidiform mole contains many cysts (sacs of fluid). it is usually benign (not cancer) but it may spread to nearby tissues (invasive mole). it may also become a malignant tumor called choriocarcinoma. hydatidiform mole is the most common type of gestational trophoblastic tumor. also called molar pregnancy.